cmd/go: call flag.Parse to properly initialize test environment variables
Executing tests in src/cmd/go/internal/modfetch/codehost/git_test.go in enviroment
witout outside connectivity in to the internet results in tests failure:
2018/07/13 14:31:14 git clone --mirror https://vcs-test.golang.org/git/gitrepo1 /tmp/gitrepo-test-
996701800/gitrepo2 in : exit status 128:
Cloning into bare repository '/tmp/gitrepo-test-
996701800/gitrepo2'...
fatal: unable to access 'https://vcs-test.golang.org/git/gitrepo1/': Could not resolve host: vcs-test.golang.org
FAIL cmd/go/internal/modfetch/codehost 0.307s
Fixes #26007
Change-Id: Ia39d8b3215c920dad6c0c58ffabb0b2ab39bb55c
Reviewed-on: https://go-review.googlesource.com/123735
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>