test-driver: Implement debugging breakpoint hooks

Co-authored-by: Maximilian Bosch <maximilian@mbosch.me>

nixos/lib/test-script-prepend.py

@@ -1,6 +1,7 @@
 # This file contains type hints that can be prepended to Nix test scripts so they can be type
 # checked.
 
+from test_driver.debug import DebugAbstract
 from test_driver.driver import Driver
 from test_driver.vlan import VLan
 from test_driver.machine import Machine
@@ -52,4 +53,5 @@ join_all: Callable[[], None]
 serial_stdout_off: Callable[[], None]
 serial_stdout_on: Callable[[], None]
 polling_condition: PollingConditionProtocol
+debug: DebugAbstract
 t: TestCase