Document the requirements for our container testing system:
- uid-range
- auto-allocate-uids
Further document that the container tests are used by default and how to
switch to the more traditional and more supported / featureful VM
testing framework.
Add documentation explaining that services defining vars need to run
update-vars before tests can execute. Clarifies how `clan.directory`
determines where vars are generated and loaded from during testing.
Closes: #5380
