improve groovy test

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1437746 13f79535-47bb-0310-9956-ffa450edef68
add verify

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1437747 13f79535-47bb-0310-9956-ffa450edef68
[MCOMPILER-199] Plugin 3.0 does not compile src/main/groovy if neither src/main/java nor src/main/resources folders exist

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1437748 13f79535-47bb-0310-9956-ffa450edef68