SSH keys for users were previously defined in ssh_keys option.
They can nowadays also be defined in ssh_keys_by_forest.
At least one of those is mandatory.
Commit 105e8404727a introduced that mechanism, but tests weren't updated.
Commit 776470ac99d2 was the first to use ssh_keys_by_forest
and so broke this test suite.
This is a follow-up for 105e8404727a.
Ref T2123