Make MemoryAssert.assertGC stricter when soft references are disallowed

[dwnusbaum]

While testing jenkinsci/pipeline-groovy-lib-plugin#199, I ran into cases where MemoryAssert.assertGC(..., false) was passing, but very slowly, because although there were soft references, they were cleared out prior to an OutOfMemoryError occurring, and so we skipped the code that failed when softErr != null.

This PR adjusts the code to fail if softErr != null after existing the initial loop, so that we also fail in this kind of case.

I have not attempted to test this against other callers of this method, so this is just a draft PR.

Testing done

See the new test in jenkinsci/pipeline-groovy-lib-plugin#199.

Submitter checklist

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• Ensure that the pull request title represents the desired changelog entry
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• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[jglick]

OK I think

[dwnusbaum]

Suggested change

	* <p>Beware that failures messages can be misleading and you may need to use heap dumps to diagnose issues.
	* <p>Beware that failure messages can be misleading, and you may need to use heap dumps to diagnose issues.