CI: Add a cache policy for test jobs and fix rum tests recompiling

everything

.gitlab-ci.yml

 image: elixir:1.8.1
 
-variables:
+variables: &global_variables
   POSTGRES_DB: pleroma_test
   POSTGRES_USER: postgres
   POSTGRES_PASSWORD: postgres
   DB_HOST: postgres
   MIX_ENV: test
 
-cache:
+cache: &global_cache_policy
   key: ${CI_COMMIT_REF_SLUG}
   paths:
           - deps
@@ -46,6 +46,10 @@ benchmark:
 
 unit-testing:
   stage: test
+  cache: &testing_cache_policy
+    <<: *global_cache_policy
+    policy: pull
+
   services:
   - name: postgres:9.6
     alias: postgres
@@ -58,6 +62,7 @@ unit-testing:
 
 federated-testing:
   stage: test
+  cache: *testing_cache_policy
   services:
   - name: minibikini/postgres-with-rum:12
     alias: postgres
@@ -71,11 +76,13 @@ federated-testing:
 
 unit-testing-rum:
   stage: test
+  cache: *testing_cache_policy
   services:
   - name: minibikini/postgres-with-rum:12
     alias: postgres
     command: ["postgres", "-c", "fsync=off", "-c", "synchronous_commit=off", "-c", "full_page_writes=off"]
   variables:
+    <<: *global_variables
     RUM_ENABLED: "true"
   script:
     - mix deps.get
@@ -86,17 +93,20 @@ unit-testing-rum:
 
 lint:
   stage: test
+  cache: *testing_cache_policy
   script:
     - mix format --check-formatted
 
 analysis:
   stage: test
+  cache: *testing_cache_policy
   script:
     - mix deps.get
     - mix credo --strict --only=warnings,todo,fixme,consistency,readability
 
 docs-deploy:
   stage: deploy
+  cache: *testing_cache_policy
   image: alpine:latest
   only:
   - stable@pleroma/pleroma