Hide empty packages on junit output

[flowchartsman]

Followup to #282 and #283. I am leaving the failed golden test in here, because I think perhaps it might be a bug. The original golden test output for junit.xml includes the following section, which my code hides:

<testsuite tests="0" failures="0" time="0.001000" name="gotest.tools/gotestsum/testjson/internal/badmain" timestamp="0001-01-01T00:00:00Z">
	<properties>
		<property name="go.version" value="go7.7.7"></property>
	</properties>
	<testcase classname="" name="TestMain" time="0.000000">
		<failure message="Failed" type="">sometimes main can exit 2&#xA;FAIL&#x9;gotest.tools/gotestsum/testjson/internal/badmain&#x9;0.001s&#xA;</failure>
	</testcase>
</testsuite>

This looks wrong to me, since tests="0" and failures="0", yet there's clearly a failing testcase being simulated. Not sure if it has to do with the empty classname or if a failure like this is likely to occur or not, but it could conceivably get eaten by a test suite during display. Not sure how the different libraries handle having both values set to zero and still having a non-skipped test case, but I definitely want to confirm before I tell remove it from the golden file.

Also worth noting: I also tried len(pkg.Passed) ==0 && len(pkg.Failed)==0 && len(pkg.Skipped)==0 and no change.

[dnephin]

Thank you for the PR! This looks great!

It was a good call to keep that failing test around. The scenario this is testing is when a package has a TestMain function or init function. Those functions can fail or panic, and when they do the count of tests will be 0, but not because the package was empty. The count is 0 because none of the tests in the package ran (due to the earlier failure from TestMain or init).

I fixed that by checking Package.TestMainFailed(). When that is true it's likely the test count is empty because of the earlier failure.

I made a small change to the field name in junitxml.Config, to make it consistent with the other field name and the command line flag. I updated the expected value for the help output (to fix a test failure), and adjusted the help text slightly.

I went back and checked the changes from #283, to see if they also need to use this new IsEmpty function. Two of them seemed fine as-is because they only hide output when the package ends in Skip or Pass. We know there are no TestMain failures in those cases (the package will always end with Fail if there are TestMain failures). Only the dots-v2 format needed a fix, so I fixed that in another commit, which I also pushed to this PR.

If CI is happy with these changes, I think this is ready to merge.