Fix uniqueness checker to anchor grep to line-start class definitions

grep class matched issubclass() calls too, causing false duplicate
detection when test_parse_strict.py was added alongside test_file_size.py
(both assert issubclass(..., DeprecationWarning)). Changing to grep "^class"
restricts matching to actual class definition lines only.

Author: timlnx

tests/test_unique_testcase_names.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-grep class tests/*.py | awk '{
+grep "^class" tests/*.py | awk '{
     arr[$NF]++
   }
   END {