Add simple "grep" to hide the harmless "warning: no packages being tested depend on ..." in "go test -coverpkg ..." output

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4283 from tianon/hide-no-packages-being-tested

Add simple "grep" to hide the harmless "warning: no packages being tested depend on ..." in "go test -coverpkg ..." output