Add integration tests to check Gradle plugin's war packaging behaviour

There was a problem with the Gradle plugin packaging application
dependencies in WEB-INF/lib-provided when only the servlet container
and its dependencies should be packaged there. See #1064 for details.

This commit adds two tests, one for Tomcat and one for Jetty, to
verify that only the expected entries appear in WEB-INF/lib-provided.

Closes #1071