bitstring_test: fix bit_nclear/bit_nset tests for last iteration

Also convert nearby ATF_REQUIRE_MSG to ATF_REQUIRE_INTEQ_MSG.

Reported by: GCC -Wint-in-bool-context
Reviewed by: asomers, des
Differential Revision: https://reviews.freebsd.org/D45893

(cherry picked from commit 54b24b9da8be70192b7baaeae8fc26fda6581222)