#15334 introduced a bug where HomeServer instances no longer get garbage collected during trial test runs. As a result, memory usage grows to multiple gigabytes during test runs.

See https://github.com/matrix-org/synapse/pull/15334/files#diff-f50ec854a65082453f4086048f20c291ca6020fab973486a9da2285025394cc8R180.
When we garbage collect generation 0, self.hs still holds a reference to the HomeServer, which gets promoted to generation 1 and then never collected.

Apart from that, there are also a couple of other leaks at play.

• Fix HomeServers leaking due to not being GCed.
  Fix HomeServers leaking during trial test runs #15630
• Fix FederationRateLimiter (and referenced HomeServers) leaking due to _rate_limiter_instances global.
  Fix HomeServers leaking during trial test runs #15630
• Fix config cache added in Speedup tests by caching HomeServerConfig instances #15284 growing too large (~360 entries). HomeServerConfigs are pretty heavy and hold on to a lot of stuff, including Jinja templates. A suitable max size for the cache might be 8-16 entries.
  Limit the size of the HomeServerConfig cache in trial test runs #15646
• Fix HomeServers leaking via synapse.util.caches.caches_by_name.
• Fix HomeServers leaking via synapse.util.caches.CACHE_METRIC_REGISTRY.
• Fix HomeServers leaking via synapse.config.cache._CACHES.

The last 3 are fairly annoying to fix, so I've left them alone.