Some people on the forum say you should notify IRCC, some say it is not necessary. I can't find an IRCC policy document that says something one way or another.
It is unlikely that your file would make it to the point where you'll get a test invitation. But, should that (unexpectedly swiftly) happen then you would be invited to a test with a week or two, probably three, notice. If that clashes with your travel plans: being abroad seems like a valid reason to reschedule your appointment, so it should not pose much of a problem. It may cause a bit of a delay, as rescheduling sometimes seems to take some time. So, you could just wait and see (and if someone can keep an eye on your mail that'd be good, so you can reschedule without delay).
In theory you can use the web form to contact IRCC, but that does not seem to be a very fast way of contacting IRCC (response time seems to be around six weeks). If you know which office will be processing your application you can send them a letter to inform them you'll be out of the country. If you do not know which office is processing your application then you can call IRCC and ask the agent to add a note to your file.
If you're heading out soon then it seems unnecessary to bother, though. If your trip is weeks away from starting then notifying IRCC may make more sense.
