Add a new integration test to verify the signal handler validation for
Type=notify-reload services, as introduced in the previous commit.
The test adds a shell harness and four service files to cover all scenarios:
1. `notify-reload-no-handler.service`: Verifies that the service fails to
start with Result=protocol when the handler is missing at the first
READY=1.
2. `notify-reload-sigstop.service`: Verifies that SIGSTOP is exempt from
handler validation because it cannot be caught or blocked.
3. `notify-reload-toggle-handler.service`: Verifies that removing the
handler at runtime results in a warning on reload, but the signal is still
sent, causing the service to terminate from unhandled SIGHUP.
4. `notify-reload-well-behaved.service`: Verifies the happy path where a
service with a handler starts and reloads successfully.
Use bounded FIFO operations and journal cursors so failures are prompt and log
assertions cannot race a relative time window.
41c12e32ce