libc: Fix TESTSDIR for new stdbit tests

Otherwise the directory created by etc/mtree/BSD.tests.dist, which is
where these belong, and referred to by the generated Kyuafile for
/usr/tests/lib/libc (via stdbit's existence in TESTS_SUBDIRS), ends up
empty with no Kyuafile, which is an error for kyua.

Reported by: kp
Fixes: 2fb8cbc6ef1b ("libc/tests: add stdbit test framework and unit tests")