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 :)
Avoid blocking at the test.
```go
select {
case <-c:
case <-time.After(Reasonable timeout here):
}
```
You might also want to check the value received from the channel.