Run all go tests in parallel (6x speedup)

Currently hack/test-go.sh runs go test in serial.  By running all tests at once, we get parallel execution.
However, we cannot use -coverprofile with this mode, which seems worthwhile (the aggregated profile is not useful).  You still get coverage %, just not a profile output

On a 4-core mac, runs tests in 15s that ran in 80s before. Tested on F20 and OSX Mavericks.
I believe all test output is shown at once, so it'd be right under the line indicating the run (that's not output until after the runs complete).