In hack/test-go.sh, treat arguments that start with a dash as go flags.

This fixes `hack/test-go.sh pkg/apiserver -test.run=<a_specific_test_name>`
which was broken by PR #1116.

@thockin 
@smarterclayton 

While technically both work here, I think "+" without the ":" is slightly more correct here, as "+" means "when undefined" and ":+" means "when empty or undefined."

For instance ":+" triggers for testcases=("") while "+" doesn't, it only triggers for testcases=().