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

[markj]

Summary

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").

[dougm]

tests/sys/sys/bitstring_test.c

339

If start != found, then not "Found %d set bits at %d", but rather "Did not find %d clear bits at %d".

353

Did not find %d clear bits at %d.

[markj]

Fix up test failure messages

[dougm]

tests/sys/sys/bitstring_test.c

353

My bad. The error is that we *did* find %d clear bits at %d, when we did not expect to.

[markj]

Fix up test failure messages, take two