[bugfix] fix case of failed timeline preload causing lockups (#4182)

- moves preloader Done() function calling to be handled entirely by the preloader, not the caller
- adds tests for multiple preload success / failure / clear states

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4182
Co-authored-by: kim <grufwub@gmail.com>
Co-committed-by: kim <grufwub@gmail.com>

internal/cache/timeline/status.go

@@ -196,14 +196,9 @@ func (t *StatusTimeline) Preload(
 	n int,
 	err error,
 ) {
-	t.preloader.CheckPreload(func(ptr *any) {
+	err = t.preloader.CheckPreload(func() error {
 		n, err = t.preload(loadPage, filter)
-		if err != nil {
-			return
-		}
-
-		// Mark as preloaded.
-		t.preloader.Done(ptr)
+		return err
 	})
 	return
 }