Feature Request: Allow adding custom properties to <testcase>

[eps1lon]

Similar to custom <properties> in <testsuite>, this would allow adding properties to <testcase>. This would allow adding metadata to individual tests in DataDog (see https://docs.datadoghq.com/continuous_integration/tests/junit_upload/?tab=jenkins#providing-metadata-through-property-elements).

The main motivation is adding the number of test invocations (see #157).

I understand that <properties> inside <testcase> would violate standard JUnit XSD but seems like this package is open to adding non-standard XML (e.g. #228).

I already have a patch locally that we're using at Klarna and will propose it as a PR once we've tested it internally.

[palmerj3]

Nice!

I'll happily review any PRs I see. So long as non-standard XML is gated behind a configuration option I'm fine with it.

[eps1lon]

@palmerj3 Opened #247 which needs approval to run CI.