tests/sys/opencrypto/runtests: apply minor polish to test script

tests/sys/opencrypto/runtests: apply minor polish to test script

- Refactor kld loading/unloading logic:
-- Use a loop instead of an unrolled one.
-- Check for the module being loaded before trying to load it, to reduce
   noise when loading modules that are already loaded.
-- Don't mute stderr from kldload -- it could be potentially useful to
   the tester.
-- In the event that the test script was terminated early, it would leave
   the modules still attached to the system (which is undesirable).
   Always unload the modules at test end with EXIT/SIGINT/SIGTERM so the
   system is returned to its former operating state as best possible.
   Unload the modules in reverse order, in part for consistency and/or
   dependency reasons.

MFC after:	2 weeks
Sponsored by:	Dell EMC Isilon