Add an option to sub_test to warn if a test took over X seconds to execute
Add an option to sub_test to warn if a test took over X seconds to execute

This functionality is motivated by trying to find stress tests for which guard
pages should be turned off, and applications that may need a higher timeout as
a result of increased execution time from guard pages.

I think it is useful beyond that too, and can easily be extended to report long
compilations times as well.

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