nstrftime tests: Allow reuse with another function.
* tests/test-nstrftime.h: New file, extracted from
tests/test-nstrftime.c.
(FUNC_NAME, STRINGIFY): New macros.
(posixtm_test, tzalloc_test, quarter_test, errno_test): Use FUNC or
FUNC_NAME instead of nstrftime.
(quarter_test): Rename local variable 'result' to 'fail'.
* tests/test-nstrftime.c: Remove code that moved to test-nstrftime.h.
Include test-nstrftime.h.
(STREQ): Remove redundant macro.
* modules/nstrftime-tests (Files): Add tests/test-nstrftime.h.