esp_system: Update task watchdog unit tests and example

This commit does the following:

- Update existing unit tests that use the TWDT to call the new
  esp_task_wdt_init() API
- Add a set of dedicate TWDT unit tests
- Updates the TWDT example

examples/system/task_watchdog/pytest_task_watchdog.py

@@ -9,7 +9,4 @@ from pytest_embedded import Dut
 @pytest.mark.generic
 def test_task_watchdog(dut: Dut) -> None:
 
-    dut.expect_exact('Initialize TWDT')
-    dut.expect_exact('Delay for 10 seconds')
-    dut.expect_exact('Unsubscribing and deleting tasks')
-    dut.expect_exact('Complete')
+    dut.expect_exact('Example complete')