selftests: forwarding: Add a test for testing lib.sh functionality

Rerunning various scenarios to make sure lib.sh changes do not impact the
observable behavior is no fun. Add a selftest at least for the bare basics
-- the mechanics of setting RET, retmsg, and EXIT_STATUS.

Since the selftest itself uses lib.sh, it would be possible to break lib.sh
in such a way that invalidates result of the selftest. Since the metatest
only uses the bare basics (just pass/fail), hopefully such fundamental
breakages would be noticed.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/6d25cedbf2d4b83614944809a34fe023fbe8db38.1711464583.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>