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, 3 insertions, 4 deletions

diff --git a/run_tests.py b/run_tests.py
index b6f8884..20edf93 100755
--- a/run_tests.py
+++ b/run_tests.py
@@ -261,11 +261,10 @@ def ensure_backend_detects_changes(backend: Backend) -> None:
         time.sleep(1)
 
 def run_mtest_inprocess(commandlist: T.List[str]) -> T.Tuple[int, str, str]:
-    stderr = StringIO()
-    stdout = StringIO()
-    with mock.patch.object(sys, 'stdout', stdout), mock.patch.object(sys, 'stderr', stderr):
+    out = StringIO()
+    with mock.patch.object(sys, 'stdout', out), mock.patch.object(sys, 'stderr', out):
         returncode = mtest.run_with_args(commandlist)
-    return returncode, stdout.getvalue(), stderr.getvalue()
+    return returncode, stdout.getvalue()
 
 def clear_meson_configure_class_caches() -> None:
     compilers.CCompiler.find_library_cache = {}