[3.14] gh-133532: Run GC fast cycles test in subprocess. (gh-133533) (gh-133716)

This makes the test more reliable since there are not extra objects on the heap leftover
from other tests.

(cherry picked from commit 8598e57942f26ca1feecc0aefb421935e4e9f2aa)

Co-authored-by: Neil Schemenauer <nas-github@arctrix.com>