Allow parser tests to be run in any order
Having a player ID which increments throughout script execution does not
allow individual parser tests to be run with --regex. Also, my pending
parser test engine refactor changes the order in which the tests are
run and so breaks tests. Instead, add a hook which resets the serial
at the start of each test.
(cherry picked from commit 8b8b191624591c0f4d447cb8c6634a9dde39ffd2)