Resolve "Make gitlab run PHPunit tests"

Closes #16

.gitlab-ci.yml

 stages:
+- unit_tests
 - marcus_test
-
 marcus_test:
   variables:
     MARCUS_SERVER: $MARCUS_TEST_SERVER_IPV6
@@ -18,3 +18,14 @@ marcus_test:
   tags:
   - spes
 
+unit_tests:
+  stage: "unit_tests"
+  image: "php:7.4"
+  before_script:
+  - apt-get update -yqq
+  - apt-get install -y git
+  - apt-cache search composer
+  script:
+  - phpunit tests/*
+  tags:
+  - spes