ostree: move test server to separate package

Make it reusable for other packages

internal/ostree/mock_ostree_repo/mock_repo.go

@@ -0,0 +1,37 @@
+package mock_ostree_repo
+
+import (
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+)
+
+type OSTreeTestRepo struct {
+	OSTreeRef string
+	Server    *httptest.Server
+}
+
+func (repo *OSTreeTestRepo) TearDown() {
+	if repo == nil {
+		return
+	}
+	repo.Server.Close()
+}
+
+func Setup(ref string) *OSTreeTestRepo {
+	repo := new(OSTreeTestRepo)
+	repo.OSTreeRef = ref
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("/refs/heads/"+ref, func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprint(w, "02604b2da6e954bd34b8b82a835e5a77d2b60ffa")
+	})
+	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		// catch-all handler, return 404
+		http.NotFound(w, r)
+	})
+
+	repo.Server = httptest.NewServer(mux)
+
+	return repo
+}