vnet tests: verify that we can load if_epair and if_bridge

We're going to start running many of the vnet tests in nested jails (so they
can run in parallel). That means the tests won't be able to load kernel modules,
which we commonly do for if_epair and if_bridge.

Just assume that all vnet tests need this, because so many of them do that we
don't want to manually annotate all of them.
This is essentially a no-op on non-nested tests.

Do the same for the python test framework.

While here also have pflog_init actually call pft_init. While having pflog
loaded implies we have pf too pft_init also checks for vimage support, and now
for if_epair.

Reviewed by: markj
MFC after: 1 month
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46039

(cherry picked from commit ae8d58814089308028046ac80aeeb9cbb784bd0a)