Tests: environment: use a context manager to restore original signal handler

Tests: environment: use a context manager to restore original signal handler

The original signal handler for SIGUSR1 was not restored when launching
the session daemon. This didn't cause any issue, but is unexpected and
would have been a real head scratcher if we wanted to use it later.

To prevent us from forgetting to restore signals when using a
_SignalWaitQueue, a context manager is provided to handle that
automatically.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I86599846ad2000af5d1e3c5cb2a0a73fb1c91455