Give directions on attaching gdb to run-test target

Attaching to run-test on target requires the use of the gdbclient.py
program and can be difficult to remember. This makes run-test print a
reminder when you run --gdb on target.

Test: none.
Change-Id: I685c4ed5b5ca19fced183e57efa04b2dca057685
1 file changed