Moves all tests into a separate package. This does not change existing
supported means of running tests (e.g. through run_tests.py).

On Mac and Windows, the default directory name conflicts with the BUILD
file we have.

Current latency profiles have their tails dominated by writing latency
logs, which is hugely undesirable.

Now when a thread log fills up, push it to a background thread to write
to disk. At shutdown, wait for all latency traces to be flushed.

It was causing git to ignore everything within src/core

Also updates the version of grpcio_test. Now that grpcio and its tests
are in different project packages, the version numbers should be kept in
sync.

Conditionally runs the tests depending on the availability of Python
versions (because Travis is the worst).

The app doesn’t do anything yet

By default don't re-run tests that have already run successfully, to
reduce latency between making a change and seeing if it fixed the last
thing that you were trying to fix.

Allows us to collect profiles of the open source build using gprof.