Handle ssh_keys_by_forest in pillar users test

Summary:
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.

Test Plan: (cd _tests && python -m unittest discover pillar)

Reviewers: DorianWinty

Differential Revision: https://devcentral.nasqueron.org/D3659