Summary:
Move tests to where we are used to them.
Note that I don't move comprehensive tests to the `tests` directory, since
I think that having them not run when `./run-tests.py` is run without arguments
may be a useful thing. In general, we may adopt this concept for the core as well.
If we decide we do want to move them, it's very easy to do.
Depends on D6719879
Test Plan: - fixups and test runs follow in the stack
Reviewers: #sourcecontrol
Differential Revision: https://phabricator.intern.facebook.com/D6719883
