Let coverage and timeout be overriden in hack/test-go.sh

As usual, shell review / test gratefully accepted.  Tested on OSX and
F20
Merge pull request #952 from smarterclayton/let_timeout_be_overriden

Let coverage and timeout be overriden in hack/test-go.sh
Remove a duplicate flag.
Merge pull request #953 from brendandburns/sanity

Remove a duplicate flag.