Add tests for install-plugins.sh

Use bats-assert and bats-support for testing
diff --git a/tests/install-plugins/Dockerfile b/tests/install-plugins/Dockerfile
index 72d28f7..ad1db16 100644
--- a/tests/install-plugins/Dockerfile
+++ b/tests/install-plugins/Dockerfile
@@ -1,3 +1,3 @@
 FROM bats-jenkins
 
-RUN /usr/local/bin/install-plugins.sh maven-plugin@2.7.1
+RUN /usr/local/bin/install-plugins.sh maven-plugin:2.7.1