Improve pkg/registry/service code coverage.

As per #917, I'm trying to learn kubernetes code base by improving pkg/registry/service code coverage.

This commit increase coverage of the package to around 81%.  There are quite a few small branch tests that accumulate to 10%+, I don't if it worth the effort to test.

One more thing about the function TestServiceRegistryList, do I need to test various cases for different labels? since now it only covers labels.Everything()...  It might be necessary unless it is covered by integration test.

Any comments are welcome :)
you might want to check the len of sl.Items first. avoid unwanted bad memory access.