Refactor UI-based tests to avoid duplication in pom.xml settings

`com.google.cloud.tools.eclipse.integration.appengine` and `com.google.cloud.tools.eclipse.appengine.newproject.test` both run tests that require the Eclipse workbench to be created, therefore both module's pom.xml contains the same configuration for `tycho-surefire-plugin` and almost identical target platform definition.

We should clean this up. Possible solutions:
- define an eclipse feature that contains the TP and just refer to that from the bundles that require it
- create a parent maven module, whose pom.xml contains these settings and any module that needs it would be a child of it thus inheriting the configuration