get external-lookup tests running in Gitlab CI

I was going to write a docker-compose thing for this to run postgres
alongside the Go tests, but then saw that Gilab has this handy-dandy
notion of a service, so used that.

.gitlab/.gitlab-ci.yml

@@ -10,6 +10,7 @@ include:
       - key: $CI_COMMIT_REF_SLUG
         paths:
           - .go/pkg/mod/
+
 variables:
   GOVERSION: "1.16.10"
 
@@ -220,6 +221,22 @@ clustertests:
   script:
     - make clustertests
 
+external lookup tests:
+  stage: integration
+  image: golang:$GOVERSION
+  variables:
+    POSTGRES_DB: $POSTGRES_DB
+    POSTGRES_USER: $POSTGRES_USER
+    POSTGRES_PASSWORD: $POSTGRES_PASSWORD
+    POSTGRES_HOST_AUTH_METHOD: trust
+  services:
+    - postgres:13.5
+  script:
+    - apt-get update --allow-releaseinfo-change -y
+    - apt-get install -y postgresql-client
+    - go test . -run "^TestExternalLookup" -externalLookupDSN postgresql://$POSTGRES_USER:$POSTGRES_PASSWORD@postgres/$POSTGRES_DB?sslmode=disable
+
+
 smoke test:
   stage: integration
   image: registry.gitlab.com/gitlab-org/cloud-deploy/aws-base:latest