internal/fetch: update _ directory test for extractReadme

Update the unit test to have an explicit _ test case that is
distinct from the stdlib case and rename the stdlib case.

Change-Id: If0d2f35c66ae42666fb773d50cc58bcae8b0a5ac
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/529376
kokoro-CI: kokoro <noreply+kokoro@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/internal/fetch/readme_test.go b/internal/fetch/readme_test.go
index ba697a3..affcead 100644
--- a/internal/fetch/readme_test.go
+++ b/internal/fetch/readme_test.go
@@ -35,7 +35,7 @@
 		want                      []*internal.Readme
 	}{
 		{
-			name:       "README at root and README in unit",
+			name:       "README at root and README in unit and README in _",
 			modulePath: stdlib.ModulePath,
 			version:    "v1.12.5",
 			want: []*internal.Readme{
@@ -47,13 +47,10 @@
 		},
 		{
 			name:       "directory start with _",
-			modulePath: stdlib.ModulePath,
-			version:    "v1.12.5",
-			want: []*internal.Readme{
-				{
-					Filepath: "cmd/pprof/README",
-					Contents: "This directory is the copy of Google's pprof shipped as part of the Go distribution.\n",
-				},
+			modulePath: "github.com/my/module",
+			version:    "v1.0.0",
+			files: map[string]string{
+				"_foo/README.md": "README",
 			},
 		},
 		{