fix: remove invalid trailing line from Dockerfile.test

My own previous trivial-touch commit appended a bare '-- ...' line
without a # prefix, which isn't valid Dockerfile syntax and broke the
buildx parse ('unknown instruction: --') - explains why the last CI
run died immediately again despite the actual fix (corrected password)
being in place.

Dockerfile.test

@@ -11,4 +11,3 @@ COPY . .
 RUN --mount=type=secret,id=test_db_url,required=true \
     export TEST_DATABASE_URL="$(cat /run/secrets/test_db_url)" && \
     cargo test -p db --test ai_credits --test ai_credits_reaper -- --test-threads=1
--- verified working: see docs/LIVE_SERVER_RUNBOOK.md step 6