frexp* tests: Refactor. * tests/test-frexp.h: New file, extracted from tests/test-frexpl.c. * tests/test-frexp.c: Include and use it. * tests/test-frexpf.c: Likewise. * tests/test-frexpl.c: Likewise. * modules/frexp-tests (Files): Add tests/test-frexp.h. * modules/frexpf-tests (Files): Likewise. * modules/frexpl-tests (Files): Likewise.