Add query test script and test mode

* Add a mode to test all metric queries

* Add a script to run query tests against different versions of
  PostgeSQL

* Add Docker elements for query testing

* Switch to using a --config flag when specifying the config file

* Fix some metric queries

* Allow the agent address to be configured

* Allow the sslmode connection parameter to be configured

tests/Dockerfile

@@ -0,0 +1,22 @@
+FROM alpine:3.21
+
+RUN apk update && \
+    apk add py3-psycopg2 \
+            py3-yaml \
+            tini
+
+WORKDIR /app
+
+COPY src/pgmon.py /app/
+
+COPY sample-config/pgmon-metrics.yml /app/
+
+COPY tests/test-config.yml /app/
+
+COPY --chmod=0600 --chown=postgres:postgres tests/pgpass /root/.pgpass
+
+ENTRYPOINT ["tini", "--"]
+
+EXPOSE 5400
+
+CMD ["/app/pgmon.py", "-c", "/app/test-config.yml", "--test"]