OptimisingTestSuite is a bit wasteful with @skip-decorated tests

[Free Ekanayaka]

If a test is decorated with the @unittest.skip, we know upfront that is not going to be run. Still OptimisingTestSuite builds the resources for it. I think there's room for being a bit more smart here.

Take the attached sample script (run it with "python test_foo.py"), you'll see that "make some resource" gets printed in the standard output even if the test is skipped altogether.

I'm attaching a sketch of a patch that would improve the situation, for these cases that we already know that the test will be skipped. If you think it's a viable solution I might port it to a proper branch with tests.

[Free Ekanayaka (free.ekanayaka)]

Sketch patch for avoiding building resources unnecessarily when we know upfront that a test will be skipped.

[Robert Collins (lifeless)]

That sketch looks viable. However, rather than peeking under the hood, lets add a helper function to unittest to query for these statically determinable things - then we can consume that from unittest2.