Ban JUnit 4 imports (jenkinsci#258)

Plugin tests have switched to JUnit 5.  Prevent introduction of new
JUnit 4 tests.

Author: MarkEWaite

pom.xml

@@ -48,6 +48,7 @@
     <spotbugs.effort>Max</spotbugs.effort>
     <spotbugs.threshold>Low</spotbugs.threshold>
     <spotless.check.skip>false</spotless.check.skip>
+    <ban-junit4-imports.skip>false</ban-junit4-imports.skip>
   </properties>
 
   <dependencies>