Enhance isinf tests.
Enhance isinf tests.

Some tests are enhanced in order to test all classes with
positive and negative values.

Signed-off-by: Stefan Liebler <8bf38aec6ae5a260784bfd12203b32cefe7ce79a@linux.ibm.com>

gftg85/libdfp,gftg85/libdfp,gftg85/libdfp,gftg85/libdfp