Add Node.js to test containers for sexp.js parity tests

Node 20 from Debian packages — needed to run test_sexp_js.py which
verifies JS renderer output matches Python renderer output.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/Dockerfile

@@ -12,7 +12,7 @@ ENV PYTHONDONTWRITEBYTECODE=1 \
 WORKDIR /app
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
-      ca-certificates \
+      ca-certificates nodejs \
     && rm -rf /var/lib/apt/lists/*
 
 COPY shared/requirements.txt ./requirements.txt