vnet tests: check for if_bridge.ko

[kp]

Summary

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.

MFC after: 1 week
Event: Kitchener-Waterloo Hackathon 202406

[markj]

tests/sys/common/vnet.subr

34

This isn't vnet-specific so it's a bit strange to put it here, but obviously that applies to other things that are already here.

[kp]

tests/sys/common/vnet.subr

34

The functions are all intended for use with the vnet-based network tests, so that's where the name came from.
I think that that's also still the only way they're actually used.