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 +18,16 @@ marcus_test:
   tags:
   - spes
 
+unit_tests:
+  stage: "unit_tests"
+  image: "php:7.4"
+  before_script:
+  - apt-get update -yqq
+  - apt-get install -y git zip
+  - curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
+  - composer install
+  script:
+  - find -ipath "*phpunit*"
+  - ./vendor/bin/phpunit tests/*.php
+  tags:
+  - spes