[flang][tests] Fix Python bug in the lit config

Without this change LIT tests for Flang fail with:
```
TypeError: append() takes exactly one argument (2 given)
```

flang/test/lit.cfg.py

@@ -43,7 +43,8 @@ config.excludes = ['Inputs', 'CMakeLists.txt', 'README.txt', 'LICENSE.txt']
 if config.include_flang_new_driver_test:
   config.available_features.add('new-flang-driver')
 else:
-  config.excludes.append('Flang-Driver','Frontend')
+  config.excludes.append('Flang-Driver')
+  config.excludes.append('Frontend')
 
 # test_source_root: The root path where tests are located.
 config.test_source_root = os.path.dirname(__file__)