test(slideshow): drop the template cache after the hand-written bad files

The folder's mtime may not move from the save's listing to the list that follows (same clock tick on a fast machine), so the stale cache hid the bad files and problems came back empty - deterministic on a warm build. The test drops the cache as its own wild.json case already did (and as templates_changed documents).

crates/engine/src/tests_slide_template.rs

@@ -248,6 +248,9 @@ fn bad_files_are_reported_and_skipped() {
     }
     std::fs::create_dir_all(folder.join("folder.json")).unwrap();
     std::fs::write(folder.join("notes.txt"), b"ignored").unwrap();
+    // written by hand, not through LightCraft: the folder's time may not have moved since the
+    // save's listing (same clock tick), so the cached listing is dropped as a write would drop it
+    s.slide_templates_cache = None;
     let r = run(&mut s, "slideshow.templates.list", json!({}));
     let user: Vec<_> = r["templates"].as_array().unwrap().iter().filter(|t| t["builtin"] == false).collect();
     assert_eq!(user.len(), 1, "only the good one: {r}");