tests: fix regression that broke mtest_inprocess

In commit faf79f4539 we broke this utility
function by referencing a non-existent variable. Fortunately, the only
time we ever used said function was once, and that in a test case where
we tested that it raised a MesonException before the undefined variable
error could occur.

run_tests.py

@@ -297,7 +297,7 @@ def run_mtest_inprocess(commandlist: T.List[str]) -> T.Tuple[int, str, str]:
     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()
+    return returncode, out.getvalue()
 
 def clear_meson_configure_class_caches() -> None:
     CCompiler.find_library_cache = {}