Promote TcNullaryTC and TcCoercible to fast tests

I'm wondering whether it's sensible to omit so many typecheck testcases from
the default validate test target. As for instance, TcNullaryTC has been failing
since its introduction in c63a465011b99eeafbb957074e54c2e6bbf751d9 (re #8993) and
it seems to have gone unnoticed so far.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>