Pin the grace probe in the tick-driven teardown tests

Both tests need the grace loop to reach their injected tick; on a fast
runner the group can read as spent on the first probe and the loop breaks
before the tick fires. The tests' subject is the tick's effect, not
liveness, so the probe is pinned open.

Author: gorandodig

tests/test_terminal_process.py

@@ -1194,6 +1194,9 @@ def interrupting_tick():
             raise KeyboardInterrupt()
 
         process._grace_tick = interrupting_tick
+        # The subject is exception propagation, not liveness: pin the probe so the
+        # grace loop cannot break before the tick fires on a fast tree.
+        process._group_spent = lambda pgid: False
         with pytest.raises(KeyboardInterrupt):
             process.terminate_tree(grace=1.0)
     finally:
@@ -1223,6 +1226,7 @@ def failing_tick():
             real_tick()
 
         process._grace_tick = failing_tick
+        process._group_spent = lambda pgid: False  # the fault must get its tick
         process.terminate_tree(grace=0.5)
     finally:
         process.close()