Add outsuffix option

[Konboi]

I want to record test results without a datetime suffix.
Because I need to send test results as the same name to use CircleCi's test in parallel (https://circleci.com/docs/2.0/parallelism-faster-jobs/#splitting-by-timing-data)

However, the xmlrunner adds a date-time suffix to a test suite automatically. Although there are the same test cases, the CircleCI realizes different test cases.

So I add an outsuffix option to control a suffix from the command.

e.g)

xmlrunner --output ./test-results/  --outsuffix="" tests/**/test_*.py

$ xmlrunner -h
usage: xmlrunner [-h] [-o DIR | --output-file FILENAME] [--outsuffix STRING]

optional arguments:
  -h, --help            show this help message and exit
  -o DIR, --output DIR  Directory for storing XML reports ('.' default)
  --output-file FILENAME
                        Filename for storing XML report
  --outsuffix STRING    Output suffix (timestamp is default)

How about this option? Will you please review it?

[Konboi]

@dnozay I added test code. Could you review this?

[Konboi]

@dnozay or anyone What is this status...? Is there anything need to do?

[dnozay]

@Konboi , sorry, yes, could you please rebase so it runs the checks?
thank you

[Konboi]

@dnozay Thank you for your reply. I rebased.
Could you approve to run github actions?

[Konboi]

@dnozay Could you approve to run Github actions...?