Make tests platform-independent

The test suite of the plugin uses the the JVM's default locale to get its resource bundle. However, AbstractMavenReport.execute() will always use the English locale when calling MavenReport.generate(). Hence, the tests will search for say German strings in a report generated from English resources...

By the way, the current head of the maven-reporting-impl simply throws a MojoExecutionException from AbstractMavenReport.execute(). All tests will fail once the plugin updates its dependencies. Not to mention that the report cannot be generated from the command line any more, pitty.