Add Maven Plugin End-to-End Tests

[vjkoskela]

There is a test class called ProcessTestMojoTest which attempts to execute the plugin end-to-end with a pom.xml file from test/resources. Although we have used this pattern in other projects (e.g. json-schema-validator) it fails here. This issue needs to be resolved and the various tests casses for the process and test-process mojos should be added.