Don't track hidden tests in Engine.
Don't track hidden tests in Engine.

We were closing hidden tests in Engine.close(), but this isn't
necessary. A test is only hidden if it's complete, complete tests
don't block closing their parent suites, *and* closing the parent suite
will close the test anyway.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1879823002 .

kevmoo/test,grouma/test,iglherme/Guttiest,grouma/test,grouma/test,kevmoo/test,kevmoo/test,iglherme/Guttiest,iglherme/Guttiest