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.
Merge pull request #269 from ronawho/qthreads-timeouts

Add option to sub_test to warn if a test took over X seconds to execute

[reviewed by @noakesmichael]

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.

Note that this currently must be manually set by a user, and nothing in start_test 
sets it. It's also unaffected by the timeout value, it's just a simple "did this test
take over X seconds to execute?"