refactor(server/api/test): withUser helper; migrate 51 context-value sites (A3)

internal/api/radio_test.go

@@ -12,7 +12,7 @@ import (
 
 func callRadio(h *handlers, user interface{}, query string) *httptest.ResponseRecorder {
 	req := httptest.NewRequest(http.MethodGet, "/api/radio?"+query, nil)
-	req = req.WithContext(context.WithValue(req.Context(), userCtxKeyForTest(), user))
+	req = withUser(req, user)
 	w := httptest.NewRecorder()
 	h.handleRadio(w, req)
 	return w