style(scrobble): rename unused r *http.Request to _ in client tests

Caught by revive's unused-parameter check. The four error-path tests
don't read the request body, so the parameter is properly named _.

internal/scrobble/listenbrainz/client_test.go

@@ -55,7 +55,7 @@ func TestClient_SubmitListens_Success(t *testing.T) {
 }
 
 func TestClient_SubmitListens_401_ReturnsErrAuth(t *testing.T) {
-	c, srv := newTestClient(func(w http.ResponseWriter, r *http.Request) {
+	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
 		w.WriteHeader(http.StatusUnauthorized)
 	})
 	defer srv.Close()
@@ -66,7 +66,7 @@ func TestClient_SubmitListens_401_ReturnsErrAuth(t *testing.T) {
 }
 
 func TestClient_SubmitListens_400_ReturnsErrPermanent(t *testing.T) {
-	c, srv := newTestClient(func(w http.ResponseWriter, r *http.Request) {
+	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
 		w.WriteHeader(http.StatusBadRequest)
 	})
 	defer srv.Close()
@@ -77,7 +77,7 @@ func TestClient_SubmitListens_400_ReturnsErrPermanent(t *testing.T) {
 }
 
 func TestClient_SubmitListens_503_ReturnsErrTransient(t *testing.T) {
-	c, srv := newTestClient(func(w http.ResponseWriter, r *http.Request) {
+	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
 		w.WriteHeader(http.StatusServiceUnavailable)
 	})
 	defer srv.Close()
@@ -88,7 +88,7 @@ func TestClient_SubmitListens_503_ReturnsErrTransient(t *testing.T) {
 }
 
 func TestClient_SubmitListens_429_ReturnsRetryAfter(t *testing.T) {
-	c, srv := newTestClient(func(w http.ResponseWriter, r *http.Request) {
+	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
 		w.Header().Set("Retry-After", "60")
 		w.WriteHeader(http.StatusTooManyRequests)
 	})