Attach the test resources directory to the build

I wanted to attach some test input files for my unit tests, but didn't see a way to do so immediately. This patch makes the plugin attach the `test/resources` directory to the build and add it to the classpath so it can be used for tests. For example:

    InputStream in = this.getClass().getClassLoader().getResourceAsStream(filename);

I'm not too sure if this is the right way to go about this or whether it will work with all of the flavors/variants of the build.