Fix htools, QuickCheck library detection and tests

Just saw this while testing the migration to QuickCheck v2: while
configure.ac detects that QuickCheck-2.x is not available, the test in
Makefile.am was against WANT_HTOOLS (overall htools compilation), not
on a more-specific WANT_HTOOLSTESTS.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>