Run all go tests in parallel (6x speedup)

Currently hack/test-go.sh runs in serial in independent executions of
go test.  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.