tests/taskmanager: fix timing-related false error
tests/taskmanager: fix timing-related false error

The "message" and "num tasks" signals happen at different places, so
it's safest to wait for both of them to arrive.

Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui