capsicum-test: Update for O_BENEATH removal

Update the tests to check O_RESOLVE_BENEATH instead.
If this looks reasonable, I'll try to upstream this change.
This keeps a compat fallback for O_BENEATH since the Linux port still
has/had O_BENEATH with "no .., no absolute paths" semantics.

Test Plan: /usr/tests/sys/capsicum/capsicum-test -u 977 passes and

		runs the O_RESOLVE_BENEATH tests.

Reviewed By: markj
Differential Revision: https://reviews.freebsd.org/D29016