tests: fix the tests failure when having multiple displays

The test framework presumes there is only one output and only save the last output (client->output) if multiple heads are dectcted on the backend.

The test_surface_output, internal_screenshot and subsurface_z_order tests require that the client should be in the current output.

This change fixed the tests failure when the device is connected to multiple displays by moving the client to the client->output.