test: Allow connecting to a running board

test: Allow connecting to a running board

Sometimes we know that the board is already running the right software,
so provide an option to allow running of tests directly, without first
resetting the board.

This saves time when re-running a test where only the Python code is
changing.

Note that this feature is open to errors, since the user must know that
the board is in a fit state to execute tests. It is useful for repeated
iteration on a particular test, where it can save quite a bit of time.

Signed-off-by: Simon Glass <sjg@chromium.org>