math: Add GLIBC_TEST_LIBM_VERBOSE environment variable support.

Allow the libm-test-driver based tests to have their verbosity set based
on the GLIBC_TEST_LIBM_VERBOSE environment variable.  This allows the entire
testsuite to be run with a non-default verbosity.

While here check the conversion for the verbose option as well.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>