Don't allow parallel complement tests

Fixes flakiness seemingly. See matrix-org/synapse#11910

Author: kegsay

.github/workflows/tests.yml

@@ -63,7 +63,7 @@ jobs:
       # Run Complement
       - run: |
           set -o pipefail &&
-          go test -v -json -tags dendrite_blacklist ./tests/... 2>&1 | gotestfmt
+          go test -v -p 1 -json -tags dendrite_blacklist ./tests/... 2>&1 | gotestfmt
         shell: bash
         name: Run Complement Tests
         env: