Allow masking widgets in kittest snapshots (#7467)

* Closes <https://github.com/emilk/egui/issues/THE_RELEVANT_ISSUE>
* [ ] I have followed the instructions in the PR template

crates/egui_kittest/tests/snapshots/test_masking.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:cdb0c0b955e3d3773a00afa61d4c44999de447aa71dd737181563101f09f5ac9
+size 5444

crates/egui_kittest/tests/tests.rs

@@ -138,3 +138,24 @@ fn test_scroll_down() {
         "The button was not clicked after scrolling down. (Probably not scrolled enough / at all)"
     );
 }
+
+#[test]
+fn test_masking() {
+    let mut harness = Harness::new_ui(|ui| {
+        let timestamp = std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH)
+            .unwrap()
+            .as_millis();
+
+        ui.label("I should not be masked.");
+        ui.label(format!("Timestamp: {timestamp}"));
+        ui.label("I should also not be masked.");
+    });
+
+    harness.fit_contents();
+
+    let to_be_masked = harness.get_by_label_contains("Timestamp: ");
+    harness.mask(to_be_masked.rect());
+
+    harness.snapshot("test_masking");
+}