Fix race condition in the language tests

When we pipe to `>(...)` like that, we unfortunately don't wait for the
process to finish. Better to just substitute the file.

Also, use the "unified" diff output that people (including myself) are
more familiar with, thanks to Git.

tests/lang.sh

@@ -52,9 +52,10 @@ for i in lang/parse-okay-*.nix; do
     i=$(basename "$i" .nix)
     if
         expect 0 nix-instantiate --parse - < "lang/$i.nix" \
-            1> >(sed "s!$(pwd)!/pwd!g" > "lang/$i.out") \
-            2> >(sed "s!$(pwd)!/pwd!g" > "lang/$i.err")
+            1> "lang/$i.out" \
+            2> "lang/$i.err"
     then
+        sed "s!$(pwd)!/pwd!g" "lang/$i.out" "lang/$i.err"
         diffAndAccept "$i" out exp
         diffAndAccept "$i" err err.exp
     else

tests/lang/framework.sh

@@ -16,7 +16,7 @@ function diffAndAcceptInner() {
     fi
 
     # Diff so we get a nice message
-    if ! diff "$got" "$expectedOrEmpty"; then
+    if ! diff --unified "$got" "$expectedOrEmpty"; then
         echo "FAIL: evaluation result of $testName not as expected"
         badDiff=1
     fi