Use bitnami/postgresql for Auto Test database

The bitnami image should be used to match the image used in Auto
Deploy.

changelogs/unreleased/use-bitnami-postgres-for-test-template.yml

+---
+title: 'Switch Auto Test CI template to `bitnami/postgresql` docker image (was previously `postgres`)'
+merge_request: 44329
+author:
+type: changed

lib/gitlab/ci/templates/Jobs/Test.gitlab-ci.yml

 test:
   variables:
-    POSTGRES_VERSION: 9.6.16
+    POSTGRES_VERSION: 9.6.16 # This is the version used by Auto Deploy by default, see https://hub.docker.com/r/bitnami/postgresql/tags for available tags
     POSTGRES_DB: test
   services:
-    - "postgres:${POSTGRES_VERSION}"
+    - name: "bitnami/postgresql:${POSTGRES_VERSION}"
+      alias: postgres
   stage: test
   image: gliderlabs/herokuish:latest
   needs: []