cmd/go/internal/modfetch: update expected tags for external repos

Updates #27692

Change-Id: Ia32b9e401dfe1fbb64b7f1311d85b7a1ab959bc0
Reviewed-on: https://go-review.googlesource.com/137775
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modfetch/coderepo_test.go

@@ -505,11 +505,11 @@ var codeRepoVersionsTests = []struct {
 	},
 	{
 		path:     "gopkg.in/russross/blackfriday.v2",
-		versions: []string{"v2.0.0"},
+		versions: []string{"v2.0.0", "v2.0.1"},
 	},
 	{
 		path:     "gopkg.in/natefinch/lumberjack.v2",
-		versions: nil,
+		versions: []string{"v2.0.0"},
 	},
 }