bitstring_test: Add regression tests for bit_ff(c|s)_area_at()

Validate the cases where a match can be found immediately and where no
match can be found. This extends the existing test cases and is enough
to catch the bug fixed in commit 6e7a585348d5 ("bitstring: fix ff_area()
when start!=0").

Reviewed by: dougm
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35259