pf tests: altq:codel_bridge requires if_bridge

Check that the bridge module is loaded before running this test.
It likely will be (as a result of running the bridge tests), but if it's
not we'll get spurious failures.

MFC after: 3 days
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit d491b42535db50693eac5946557f7527f9903b4b)