Add useJunitPlatform() to build.gradle (#1130)

This is required for the JUnit tests to run.

Source:
https://docs.gradle.org/current/userguide/java_testing.html#using_junit5

build.gradle

@@ -139,3 +139,7 @@ dependencies {
 
     developmentOnly "org.springframework.boot:spring-boot-devtools"
 }
+
+test {
+    useJUnitPlatform()
+}