Optional test for the metadata regeneration

This test changes the date of the current system before and after the
metadata regeneration command to make sure the behaviour is as expected.
It is not run by default and requires the
LTTNG_ENABLE_DESTRUCTIVE_TESTS environment variable to be set to
"will-break-my-system".

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>