A number of tests create a bridge, but did not check if if_bridge.ko is loaded.
We usually get away with that, because `ifconfig bridge create` autoloads the
module, but if we run the tests in a jail (e.g. because of kyua's upcoming
execenv.jail.params feature) we can't load the module and these tests can fail.
Check if the module is loaded, skip the test if it is not.
Reviewed by: markj
MFC after: 1 week
Event: Kitchener-Waterloo Hackathon 202406
Differential Revision: https://reviews.freebsd.org/D45487
480ad40553