Testing fixes for test_results_push.py

[ndokos]

There are some existing issues and some arising from PR #3348 that should be addressed in the pbench-results-push tests (see test_resuls_push.py):

• Some tests (e.g. the tests that test exceptions) can be subsumed under the parametrization of add_http_mock_response with suitable changes so that the latter can accept an exception as well as a string for the message). That will eliminate add_connectionerr_mock_response and the individual tests that use it.

• It might be better to specify the numeric value (400) rather than a symbolic name that happens to have the same value when deciding how to format the error message.

• See also this suggestion for modifying the parametrization of the tests.

[ndokos]

• Eliminate redundant test
• Eliminate another redundant test
• Remove comments on ll.101 and 146 of test_copy_result_tb.py