Add stage definitions of test jobs and add test to list of stages

Defining the stages instead of having travis guess them and implicitly build an order
ensures that the test stage is executed before the docker images are built.

.travis.yml

@@ -4,6 +4,7 @@ dist: focal
 os: linux
 
 stages:
+ - name: test
  - name: build_docker
    if: branch =~ /^(master|dev|ng-.*)$/
  - name: publish_manifest
@@ -12,21 +13,26 @@ stages:
 jobs:
   include:
     - name: "Paperless on Python 3.6"
+      stage: test
       python: "3.6"
 
     - name: "Paperless on Python 3.7"
+      stage: test
       python: "3.7"
 
     - name: "Paperless on Python 3.8"
+      stage: test
       python: "3.8"
 
     - name: "Documentation"
+      stage: test
       script:
         - cd docs/
         - make html
       after_success: true
 
     - name: "Front end"
+      stage: test
       language: node_js
       node_js:
         - 15