cmd/go: in tests, don't assume that the 'git' binary is present

Add a helper-function to testenv to make these skips more ergonomic.
Also update a few existing skips in cmd/go/... to use it.

Updates #25300

Change-Id: I4205b4fb2b685dfac1cff3c999f954bff7b0f3c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/181538


Reviewed-by: Ian Lance Taylor <iant@golang.org>