Error reporting for when listing tests takes forever

I also had to increase timeouts for some seemingly unrelated tests because
otherwise their listing would time out.

Before this, the test just timed out.
Make test check more generous

This CI build https://travis-ci.org/pereckerdal/overman/jobs/42765455
failed because the limits were too tight.
Increase timeouts that were too short for slow CI machines

Also improve error reporting for when these tests fail for this reason.
Merge pull request #28 from pereckerdal/test-listing-timeout-error

Good error reporting for when listing tests takes forever