cmd/go: fix TestVerifyNotDownloaded test

As CL 119475, we also need fix local cache directory root
from $GOPATH/src/v to $GOPATH/src/mod in tests.

Change-Id: I364a31adb904bf4904ad84ad5e8086b00c80ccb4
Reviewed-on: https://go-review.googlesource.com/119497
Reviewed-by: Bryan C. Mills <bcmills@google.com>
diff --git a/vendor/cmd/go/vgo_test.go b/vendor/cmd/go/vgo_test.go
index f140dc8..a2f61dd 100644
--- a/vendor/cmd/go/vgo_test.go
+++ b/vendor/cmd/go/vgo_test.go
@@ -608,8 +608,8 @@
 	tg.must(ioutil.WriteFile(tg.path("x/x.go"), []byte(`package x`), 0666))
 	tg.cd(tg.path("x"))
 	tg.run("-vgo", "mod", "-verify")
-	tg.mustNotExist(filepath.Join(tg.path("gp"), "/src/v/cache/github.com/pkg/errors/@v/v0.8.0.zip"))
-	tg.mustNotExist(filepath.Join(tg.path("gp"), "/src/v/github.com/pkg"))
+	tg.mustNotExist(filepath.Join(tg.path("gp"), "/src/mod/cache/github.com/pkg/errors/@v/v0.8.0.zip"))
+	tg.mustNotExist(filepath.Join(tg.path("gp"), "/src/mod/github.com/pkg"))
 }
 
 func TestVendorWithoutDeps(t *testing.T) {