Ensure test webjars don't include compile webjars

Previously, web-assets-test:web-jars was extracting both test webjars
and compile webjars.  This then caused a mapping error when they were
copied to the test assets target directory, because the same destination
files appeared twice with two different sources.  This ensures that
web-assets-test:web-jars only extracts the test scoped web-jars.
Merge pull request #26 from jroper/test-webjars

Ensure test webjars don't include compile webjars