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>

src/cmd/go/internal/modfetch/codehost/git_test.go

@@ -7,6 +7,7 @@ package codehost
 import (
 	"archive/zip"
 	"bytes"
+	"flag"
 	"fmt"
 	"internal/testenv"
 	"io/ioutil"
@@ -22,6 +23,9 @@ import (
 )
 
 func TestMain(m *testing.M) {
+	// needed for initializing the test environment variables as testing.Short
+	// and HasExternalNetwork
+	flag.Parse()
 	os.Exit(testMain(m))
 }