Prevent reactive-watcher loop in Tabs / TabbedContent. (#2305)

* Add regression test for #2229.

* Fix potential reactive-watch loop.

* Simplify regression test.

Labels are cheaper to use and the final visual result of the test won't depend on the directory it runs from.

* Simplify solution.

Turns out I didn't need a descriptor. :(

* Fail on empty tab.

tests/snapshot_tests/snapshot_apps/quickly_change_tabs.py

@@ -0,0 +1,24 @@
+"""Regression test for https://github.com/Textualize/textual/issues/2229."""
+from textual.app import App, ComposeResult
+from textual.widgets import TabbedContent, TabPane, Tabs, Label
+
+
+class QuicklyChangeTabsApp(App[None]):
+    def compose(self) -> ComposeResult:
+        with TabbedContent():
+            with TabPane("one"):
+                yield Label("one")
+            with TabPane("two"):
+                yield Label("two")
+            with TabPane("three", id="three"):
+                yield Label("three")
+
+    def key_p(self) -> None:
+        self.query_one(Tabs).action_next_tab()
+        self.query_one(Tabs).action_next_tab()
+
+
+app = QuicklyChangeTabsApp()
+
+if __name__ == "__main__":
+    app.run()