Consistently call the test client webservd_testc

This turns out to have interesting implications (e.g. /proc/<pid>/stat
will shorten the name of the process to fit into 15 characters if it is
too long).  webservd_test_client is too long, while webservd_testc is
not.

Bug: 26776493
Test: Built this on an emulator, brillo_WebservdSanity passes.

Change-Id: Ib25ae1ae817f204c2331a95174b76a054af48b5b