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.
how clean are the error outputs in the parallel case.  Do we want to tee them into some temporary log files?