Add NixOS test for QGIS and QGIS-LTR. This test creates QGIS vector
memory layer containing Nix snowflake. This proves that application can
successfully start and Python bindings are working.
By default, Python script is executed in non-interactive mode and QGIS
is closed after script is finished. This script can be also executed
interactively by running following command:
```
nix-build -A qgis
QGIS_TEST_INTERACTIVE=True ./result/bin/qgis --code pkgs/applications/gis/qgis/test.py
```
In this case, QGIS is not automatically closed.
