New test driver

[JukkaL]

I just merged #721 (by @o11c) which replaces the old test driver tests.py and the travis script. I'm creating this issue to increase visibility to contributors.

Summary of main workflow changes:

• Run all unit tests via ./runtests.py unit-test (this was python3 tests.py previously).
• Run all tests (similar to the previous travis script but with better coverage) via ./runtests.py.
• You can't run unit tests via python3 -m mypy.test.testcheck (etc.) any more.

The readme contains more examples of how to run tests. Also try ./runtests.py -h.

The new driver is better than the old one in several respects. These are some of biggest wins:

• Tests are run in parallel. (Though inidividual suites are still run on a single cpu and not parallelized.)
• All stubs are automatically type checked (previously some were not tested and the test configuration was more fragile).
• There is more test auto discovery.
• There is a single entry point to all tests that also supports filtering.

Test output is not great right now and needs some love, but I merged it anyway as the PR has been open for a long time and there was much grumbling (sorry!).