testing: Fix search key.

The problem this commit solves is related to how we search for
testcases in the test files. We use a simple 'search_key' in file_data.
This will return a false positive if there is a 'longer_search_key' in
file_data.

While searching for TestCases we should use a longer key to remove the
possibility of collision. Using 'class TestCase(' should be precise
enough.

Fixes #4983

tools/test-backend

@@ -236,7 +236,7 @@ if __name__ == "__main__":
             classname = suite.rsplit('.', 1)[0]
             rewrite_arguments(classname)
         elif suite[0].isupper():
-            rewrite_arguments(suite)
+            rewrite_arguments('class %s(' % (suite,))
 
     for suite in args:
         if suite.startswith('test'):