Add a test for the --vsenv meson setup option

The tests and the unittests both unconditionally call setup_vsenv()
because all tests are run using the backend commands directly: ninja,
msbuild, etc.

There's no way to undo this vs env setup, so the only way to test that
--vsenv works is by:

1. Removing all paths in PATH that provide ninja
2. Changing setup_vsenv(force=True) to forcibly set-up a new vsenv
   when MESON_FORCE_VSENV_FOR_UNITTEST is set
3. Mock-patching build_command, test_command, install_command to use
   `meson` instead of `ninja`
4. Asserting that 'Activating VS' is in the output for all commands
5. Ensure that compilation works because ninja is picked up from the
   vs env.

I manually checked that this test actually does fail when the previous
commit is reverted.

1 files changed, 10 insertions, 9 deletions

diff --git a/mesonbuild/mesonlib/vsenv.py b/mesonbuild/mesonlib/vsenv.py
index 8563b7d..fcb0c42 100644
--- a/mesonbuild/mesonlib/vsenv.py
+++ b/mesonbuild/mesonlib/vsenv.py
@@ -26,15 +26,16 @@ def _setup_vsenv(force: bool) -> bool:
         return False
     if os.environ.get('OSTYPE') == 'cygwin':
         return False
-    if 'Visual Studio' in os.environ['PATH']:
-        return False
-    # VSINSTALL is set when running setvars from a Visual Studio installation
-    # Tested with Visual Studio 2012 and 2017
-    if 'VSINSTALLDIR' in os.environ:
-        return False
-    # Check explicitly for cl when on Windows
-    if shutil.which('cl.exe'):
-        return False
+    if 'MESON_FORCE_VSENV_FOR_UNITTEST' not in os.environ:
+        if 'Visual Studio' in os.environ['PATH']:
+            return False
+        # VSINSTALL is set when running setvars from a Visual Studio installation
+        # Tested with Visual Studio 2012 and 2017
+        if 'VSINSTALLDIR' in os.environ:
+            return False
+        # Check explicitly for cl when on Windows
+        if shutil.which('cl.exe'):
+            return False
     if not force:
         if shutil.which('cc'):
             return False