go/internal/srcimporter: set -mod=vendor before running tests

Otherwise, if the working directory is inside a standard-library
module, the test may try to fetch module contents from GOPROXY or
upstream.

Updates #26924
Updates #30228
Updates #30241

Change-Id: I4cb9a07721bd808fd094f7ed55a74cf7bce9cd6f
Reviewed-on: https://go-review.googlesource.com/c/164625
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/go/internal/srcimporter/srcimporter_test.go

@@ -10,6 +10,7 @@ import (
 	"go/types"
 	"internal/testenv"
 	"io/ioutil"
+	"os"
 	"path"
 	"path/filepath"
 	"runtime"
@@ -18,6 +19,23 @@ import (
 	"time"
 )
 
+func TestMain(m *testing.M) {
+	// Add -mod=vendor to GOFLAGS to ensure that we don't fetch modules while importing std or cmd.
+	//
+	// TODO(golang.org/issue/30240): If we load go.mod files from vendor/
+	// automatically, this will probably no longer be necessary.
+	var goflags []string
+	for _, f := range strings.Fields(os.Getenv("GOFLAGS")) {
+		if !strings.HasPrefix(f, "-mod=") && !strings.HasPrefix(f, "--mod=") {
+			goflags = append(goflags, f)
+		}
+	}
+	goflags = append(goflags, "-mod=vendor")
+	os.Setenv("GOFLAGS", strings.Join(goflags, " "))
+
+	os.Exit(m.Run())
+}
+
 const maxTime = 2 * time.Second
 
 var importer = New(&build.Default, token.NewFileSet(), make(map[string]*types.Package))