Speedup tests by caching HomeServerConfig instances (#15284)

These two lines:

```
config_obj = HomeServerConfig()
config_obj.parse_config_dict(config, "", "")
```

are called many times with the exact same value for `config`.

As the test suite is CPU-bound and non-negligeably time is spent in
`parse_config_dict`, this saves ~5% on the overall runtime of the Trial
test suite (tested with both `-j2` and `-j12` on a 12t CPU).

This is sadly rather limited, as the cache cannot be shared between
processes (it contains at least jinja2.Template and RLock objects which
aren't pickleable), and Trial tends to run close tests in different
processes.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/15284.misc b/changelog.d/15284.misc
new file mode 100644
index 0000000000..99d753f8f0
--- /dev/null
+++ b/changelog.d/15284.misc
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+Speedup tests by caching HomeServerConfig instances.