Backend CI

In this merge request, an initial version of the GitLab Continuous Integration is added, which specifies jobs for the backend testing suite using Jacoco, as well as coding standards using PMD, Checkstyle and SpotBugs.

backend/build.gradle

@@ -60,6 +60,17 @@ jacocoTestReport {
     }
 }
 
+jacocoTestCoverageVerification {
+    violationRules {
+        rule {
+            limit {
+                counter = 'BRANCH'
+                minimum = 0.8
+            }
+        }
+    }
+}
+
 pmd {
     toolVersion = '6.3.0'
     consoleOutput = 'true'
@@ -79,3 +90,4 @@ tasks.withType(com.github.spotbugs.SpotBugsTask) {
 }
 
 jacocoTestReport.dependsOn test
+jacocoTestCoverageVerification.dependsOn jacocoTestReport