bpo-44895: libregrtest: refleak check clears types later (GH-28113)

libregrtest now clears the type cache later to reduce the risk of
false alarm when checking for reference leaks. Previously, the type
cache was cleared too early and libregrtest raised a false alarm
about reference leaks under very specific conditions.

Move also support.gc_collect() outside clear/cleanup functions to
make the garbage collection more explicit.

Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>