unittests: Pass the exact backend name

CI runs with vs2019 and we were passing --backend=vs. This fix
reconfigure tests because we can't reconfigure with --backend=vs when
initial configuration determined the backend is actually vs2019.

run_tests.py

@@ -358,7 +358,7 @@ def main():
     parser.add_argument('--no-unittests', action='store_true', default=False)
     (options, _) = parser.parse_known_args()
     returncode = 0
-    backend, _ = guess_backend(options.backend, shutil.which('msbuild'))
+    _, backend_flags = guess_backend(options.backend, shutil.which('msbuild'))
     no_unittests = options.no_unittests
     # Running on a developer machine? Be nice!
     if not mesonlib.is_windows() and not mesonlib.is_haiku() and 'CI' not in os.environ:
@@ -394,7 +394,7 @@ def main():
         else:
             print(mlog.bold('Running unittests.'))
             print(flush=True)
-            cmd = mesonlib.python_command + ['run_unittests.py', '--backend=' + backend.name, '-v']
+            cmd = mesonlib.python_command + ['run_unittests.py', '-v'] + backend_flags
             if options.failfast:
                 cmd += ['--failfast']
             returncode += subprocess_call(cmd, env=env)

unittests/allplatformstests.py

@@ -3460,7 +3460,7 @@ class AllPlatformTests(BasePlatformTests):
                 subsub         : YES
 
               User defined options
-                backend        : ''' + self.backend.name + '''
+                backend        : ''' + self.backend_name + '''
                 libdir         : lib
                 prefix         : /usr
                 enabled_opt    : enabled

unittests/baseplatformtests.py

@@ -55,8 +55,10 @@ class BasePlatformTests(TestCase):
         src_root = str(PurePath(__file__).parents[1])
         self.src_root = src_root
         # Get the backend
-        self.backend = getattr(Backend, os.environ['MESON_UNIT_TEST_BACKEND'])
+        self.backend_name = os.environ['MESON_UNIT_TEST_BACKEND']
-        self.meson_args = ['--backend=' + self.backend.name]
+        backend_type = 'vs' if self.backend_name.startswith('vs') else self.backend_name
+        self.backend = getattr(Backend, backend_type)
+        self.meson_args = ['--backend=' + self.backend_name]
         self.meson_native_files = []
         self.meson_cross_files = []
         self.meson_command = python_command + [get_meson_script()]