tests: be more generous with mon tests timeouts

tests: be more generous with mon tests timeouts

Change the timeouts of the TEST_mon_add_to_single_mon tests to be 120
seconds. They have value to guard against blocking forever. The exact
timing does not matter as long as the operation completes.

When a timeout is too short, it will create false negatives when running
on slow machines. It could be argued that being too generous with
timeout in general may hide problems. But not in this specific case.

Signed-off-by: Loic Dachary <ldachary@redhat.com>