Hadoop Common includes generated test protos in both jar and test-jar

Right now our ProtocMojo always adds source directories to the main compile phase and we use it in hadoop-common to both generate main files as well as test files. This results in the test files getting added to both our test jar (correct) and our main jar (not correct).

We should either add a main-vs-test flag to the configuration for ProtocMojo or make a ProtocTestMojo that always adds as a test sources.