Add test size annotation.

Rational/Background:

@MediumTest: An integration or functional test that validates the
interaction of one or more application modules on a single machine.
(should run under 1 sec: Usually get killed after 5 min)
@LargeTest: A system test, integration test, or end-to-end test that
verifies that a complete application works and accounts for the behavior
of external subsystems.
(Usually get killed after 15 min)

Change-Id: I8b668af3d2ca09c43ad45c6fd72b1d0d80c6b7d3