This change fixes two memory leaks during `trial` test runs.
Garbage collection is disabled during each test case and a gen-0 GC is
run at the end of each test. However, when the gen-0 GC is run, the
`TestCase` object usually still holds references to the `HomeServer`
used during the test. As a result, the `HomeServer` gets promoted to
gen-1 and then never garbage collected.
Fix this by periodically running full GCs.
Additionally, fix `HomeServer`s leaking after tests that touch inbound
federation due to `FederationRateLimiter`s adding themselves to a global
set, by turning the set into a `WeakSet`.
Resolves#15622.
Signed-off-by: Sean Quah <seanq@matrix.org>
