vnet tests: check for if_bridge.ko

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

(cherry picked from commit 480ad4055314efbfff7eedbb34272031d836ba89)