[testing] print test case when an Invalid test fails for easier debug

[jimmylai]

Summary

Without this change, developer needs to use the test case count (3 for INVALID_3) to figure out which test case fails. It's not convenient when the rule has lots of test cases.

FAIL: test_INVALID_3 (fixit.common.testing.NoRedundantLambdaRule)
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Traceback (most recent call last):
  File "/Users/jimmylai/github/Fixit/fixit/common/testing.py", line 221, in test_method
    return getattr(self, custom_test_method_name)(data, rule, fixture_file)
  File "/Users/jimmylai/github/Fixit/fixit/common/testing.py", line 90, in _test_method
    'Expected a report for this "invalid" test case. `self.report` was not called.',
AssertionError: 0 not greater than 0 : Expected a report for this "invalid" test case. `self.report` was not called.

With this change, the failed test case source code is included to make it more clear.

FAIL: test_INVALID_3 (fixit.common.testing.NoRedundantLambdaRule)
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Traceback (most recent call last):
  File "/Users/jimmylai/github/Fixit/fixit/common/testing.py", line 222, in test_method
    return getattr(self, custom_test_method_name)(data, rule, fixture_file)
  File "/Users/jimmylai/github/Fixit/fixit/common/testing.py", line 91, in _test_method
    + 'not called:\n' + test_case.code,
AssertionError: 0 not greater than 0 : Expected a report for this "invalid" test case but `self.report` was not called:
lambda x: fun()

Test Plan

See the above.

[zsol]

lgtm