cmd/go: call flag.Parse to properly initialize test environment variables

Executing tests in cmd/go/internal/modfetch/gitrepo/fetch_test.go in enviroment
witout outside connectivity in to the internet results in tests failure:

2018/06/25 12:48:26 git clone --mirror https://vcs-test.golang.org/git/gitrepo1 /tmp/gitrepo-test-221822392/gitrepo2 in : exit status 128:
	Cloning into bare repository '/tmp/gitrepo-test-221822392/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/gitrepo	0.144s

Call flag.Parse in TestMain to properly initialize test environment variables

Fixes #26007

Change-Id: I059e27db69c0ca0e01db724035a25d6fefb094b5
Reviewed-on: https://go-review.googlesource.com/120735
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/modfetch/gitrepo/fetch_test.go

@@ -7,6 +7,7 @@ package gitrepo
 import (
 	"archive/zip"
 	"bytes"
+	"flag"
 	"fmt"
 	"internal/testenv"
 	"io/ioutil"
@@ -23,6 +24,9 @@ import (
 )
 
 func TestMain(m *testing.M) {
+	// needed for initializing the test environment variables as testing.Short
+	// and HasExternalNetwork
+	flag.Parse()
 	os.Exit(testMain(m))
 }