complement: enable dirty runs (#16520)

* complement: enable dirty runs

* Add changelog

* Set a low connpool limit when running in Complement

Dirty runs can cause many containers to be running concurrently,
which seems to easily exhaust resources on the host. The increased
speedup from dirty runs also seems to use more db connections on
workers, which are misconfigured currently to have
`SUM(workers * cp_max) > max_connections`, causing
```
FATAL:  sorry, too many clients already
```
which results in tests failing.

* Try p=2 concurrency to restrict slowness of servers which causes partial state join tests to flake

* Debug logging

* Only run flakey tests

* Only adjust connection pool limits in worker mode

* Move cp vars to somewhere where they get executed in CI

* Move cp values back to where they actually work

* Debug logging

* Try p=1 to see if this makes worker mode happier

* Remove debug logging

.github/workflows/tests.yml

@@ -638,9 +638,10 @@ jobs:
       - name: Prepare Complement's Prerequisites
         run: synapse/.ci/scripts/setup_complement_prerequisites.sh
 
+        # use p=1 concurrency as GHA boxes are underpowered and don't like running tons of synapses at once.
       - run: |
           set -o pipefail
-          COMPLEMENT_DIR=`pwd`/complement synapse/scripts-dev/complement.sh -json 2>&1 | synapse/.ci/scripts/gotestfmt
+          COMPLEMENT_DIR=`pwd`/complement synapse/scripts-dev/complement.sh -p 1 -json 2>&1 | synapse/.ci/scripts/gotestfmt
         shell: bash
         env:
           POSTGRES: ${{ (matrix.database == 'Postgres') && 1 || '' }}