docs(test): add OIDC/MSC3861 test section + local MAS dev loop

LOTUS_TESTING.md section N (N1-N6): OIDC login flow, session-persist-on-reload,
token refresh, logout revocation, account-management link, and the non-OIDC
regression check. Backed by dev/oidc-test/ — a runnable local Matrix
Authentication Service + Synapse(msc3861) loop (compose skeleton, the Synapse
experimental_features delta, and the public/config.json override) so the flow
can be verified without a mozilla.org tester.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

dev/oidc-test/config.local.json

@@ -0,0 +1,7 @@
+{
+  "defaultHomeserver": 0,
+  "homeserverList": ["localhost:8008"],
+  "allowCustomHomeservers": true,
+  "featuredCommunities": { "openAsDefault": false, "spaces": [], "rooms": [], "servers": [] },
+  "hashRouter": { "enabled": false, "basename": "/" }
+}