Don't print test suite name if no test is being run

[alexarchambault]

These changes make munit print only the test suite names of suites for which it actually ran tests, rather than printing all suite names (even those for which all tests are being filtered out), as it's been doing up to now.

For example, instead of printing things like

$ ./mill -i -w scala.integration.test "almond.integration.KernelTestsSimple3.completion"
…
almond.integration.KernelTestsSimple3:
  + completion 17.073s
almond.integration.KernelTestsTwoStepStartup3:
almond.integration.KernelTestsSimple213:
almond.integration.KernelTestsTwoStepStartup212:
almond.integration.KernelTestsTwoStepStartup213:
almond.integration.KernelTestsSimple212:

it now prints

$ ./mill -i -w scala.integration.test "almond.integration.KernelTestsSimple3.completion"
…
almond.integration.KernelTestsSimple3:
  + completion 15.468s

I find the latter to be more readable.

[alexarchambault]

Not sure how to test that, or if it needs to be tested. (Going to see if it break things on the CI here…)

Manual tests show that it works as expected, at least.

[tgodzik]

This looks good to me, though I wonder if there was no initial purpose for printing thise? @valencik @mzuehlke any opinions here?

[mzuehlke]

I made the output of individual tests that are ignored or skipped more verbose in #766.
But for complete suites I find your change good.👍🏻
What are scalatest or junit doing in these cases?