tests/test_promise: use a larger timeout to prevent false positives

When test machine is busy it happens that running a simple promise takes more
than one second and this cause some test to error with "Promise timed out after
1 seconds" in cases where the promise is not supposed to have a timeout, it's
just that sometimes it takes more than the default timeout of 1 second.
1 job for master
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master