nixos/test-driver: drop logging from Machine.send_monitor_command

Several machine operations, like `send_chars` and `send_key`, are
implemented by calling `send_monitor_command`, possibly multiple times.
This generates a huge amount of unnecessary noise in the log, because
`send_monitor_command` is a low-level operation and an implementation
detail.

Here's an excerpt from a highlighted log before and afte the change.

Before:

    subtest: Can generate a PGP key
    machine: sending keys 'gpg --gen-key\n'
    machine: sending monitor command: sendkey g
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey g, in 0.00 seconds)
    machine: sending monitor command: sendkey p
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey p, in 0.00 seconds)
    machine: sending monitor command: sendkey g
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey g, in 0.00 seconds)
    machine: sending monitor command: sendkey spc
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey spc, in 0.00 seconds)
    machine: sending monitor command: sendkey 0x0C
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey 0x0C, in 0.00 seconds)
    machine: sending monitor command: sendkey 0x0C
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey 0x0C, in 0.00 seconds)
    machine: sending monitor command: sendkey g
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey g, in 0.00 seconds)
    machine: sending monitor command: sendkey e
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey e, in 0.00 seconds)
    machine: sending monitor command: sendkey n
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey n, in 0.00 seconds)
    machine: sending monitor command: sendkey 0x0C
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey 0x0C, in 0.00 seconds)
    machine: sending monitor command: sendkey k
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey k, in 0.00 seconds)
    machine: sending monitor command: sendkey e
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey e, in 0.00 seconds)
    machine: sending monitor command: sendkey y
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey y, in 0.00 seconds)
    machine: sending monitor command: sendkey ret
    machine: waiting for monitor prompt
    (finished: waiting for monitor prompt, in 0.00 seconds)
    (finished: sending monitor command: sendkey ret, in 0.00 seconds)
    (finished: sending keys 'gpg --gen-key\n', in 0.15 seconds)

After:

    subtest: Can generate a PGP key
    machine: sending keys 'gpg --gen-key\n'
    (finished: sending keys 'gpg --gen-key\n', in 0.15 seconds)

nixos/lib/test-driver/test_driver/machine.py

@@ -1,4 +1,4 @@
-from contextlib import _GeneratorContextManager
+from contextlib import _GeneratorContextManager, nullcontext
 from pathlib import Path
 from queue import Queue
 from typing import Any, Callable, Dict, Iterable, List, Optional, Tuple
@@ -406,7 +406,6 @@ class Machine:
         return rootlog.nested(msg, my_attrs)
 
     def wait_for_monitor_prompt(self) -> str:
-        with self.nested("waiting for monitor prompt"):
         assert self.monitor is not None
         answer = ""
         while True:
@@ -420,7 +419,6 @@ class Machine:
 
     def send_monitor_command(self, command: str) -> str:
         self.run_callbacks()
-        with self.nested(f"sending monitor command: {command}"):
         message = f"{command}\n".encode()
         assert self.monitor is not None
         self.monitor.send(message)
@@ -685,9 +683,9 @@ class Machine:
             retry(tty_matches)
 
     def send_chars(self, chars: str, delay: Optional[float] = 0.01) -> None:
-        with self.nested(f"sending keys '{chars}'"):
+        with self.nested(f"sending keys {repr(chars)}"):
             for char in chars:
-                self.send_key(char, delay)
+                self.send_key(char, delay, log=False)
 
     def wait_for_file(self, filename: str) -> None:
         """Waits until the file exists in machine's file system."""
@@ -860,8 +858,12 @@ class Machine:
                 if matches is not None:
                     return
 
-    def send_key(self, key: str, delay: Optional[float] = 0.01) -> None:
+    def send_key(
+        self, key: str, delay: Optional[float] = 0.01, log: Optional[bool] = True
+    ) -> None:
         key = CHAR_TO_KEY.get(key, key)
+        context = self.nested(f"sending key {repr(key)}") if log else nullcontext()
+        with context:
             self.send_monitor_command(f"sendkey {key}")
             if delay is not None:
                 time.sleep(delay)