Streamline the code for flagging warnings in the reports

[aloukina]

When tests are run in STRICT mode, we check for any errors or warnings in the report.

We currently have two functions:

test_utils.check_report https://github.com/EducationalTestingService/rsmtool/blob/main/rsmtool/test_utils.py#L767 checks for both errors and warnings

collect_warning_messages_from_report https://github.com/EducationalTestingService/rsmtool/blob/main/rsmtool/test_utils.py#L737 once again collects warning messages.

The warnings that are excluded under check_report would still be flagged as errors by collect_warning_messages_from_report.

We need to figure out why we have these two functions and streamline them.