This prohibits the usage of workspaces assigned to other outputs in
create_workspace_on_output.
Eg, with config:
workspace 1 output fake-0
workspace 2 output fake-0
and 2 screens workspace 2 would be used for the second screen even
though it is assigned to the first one.
Also introduces a test for workspace assignments that includes the case
described above and some tests that don't fail in the next branch.
d525eb80ae