Add script to run tests before landing changes.

Summary:
tools/test_and_submit.sh will merge your changes into master, run the test
suite on the resultant working copy, and push only if the tests pass.
Consider using it instead of `arc land` to submit changes.

Test Plan:
Checked that script aborts if the tests fail; successfully
used it to submit an accepted commit. The script probably won't handle every
weird Git edge-case though.

Reviewers: spang

Reviewed By: spang

CC: kav-ya, mg

Differential Revision: https://review.inboxapp.com/D46