Try to speed up ping-dependent capture tests.

Ping four times a second for ~60 seconds. Save the subprocess PID so that
we can kill it when we're done with each test instead of waiting for it
to finish.

Change-Id: I64f889c700e8a6fa1bc1c3916ef045341ef59cc6
Reviewed-on: https://code.wireshark.org/review/4557
Reviewed-by: Gerald Combs <gerald@wireshark.org>