rest: Fix test backend url

The rest config normally uses prepareURL to sanitize URLs and ensures
that the URL ends with a slash. However, the test used an URL without a
trailing slash, which after the rest server changes causes test
failures.

internal/backend/rest/rest_test.go

@@ -50,7 +50,7 @@ func runRESTServer(ctx context.Context, t testing.TB, dir string) (*url.URL, fun
 		return nil, nil
 	}
 
-	url, err := url.Parse("http://localhost:8000/restic-test")
+	url, err := url.Parse("http://localhost:8000/restic-test/")
 	if err != nil {
 		t.Fatal(err)
 	}