Drop uselongdouble unless it really gives longer doubles.
Perl_isfinitel is not necessarily always there.
undef the temp variables between tests.

This way a failing result doesn't carry over between subtests.
Fix one test message, add two tests.
Wrong (non-existent) Config var name.
skip_all, not skip, if only miniperl.
Using bignum needs skip_if_miniperl().