Test suite failures on docker

[ad-daniel]

These tests appear to be failing on a docker-run instance of webots, as can be seen here:

• FAILURE with pen_mesh_scaled: The pixel (46,26) should not be gray at last step
• FAILURE with distance_sensor_infra-red_vs_mesh: Distance sensor 'B' doesn't return the right distance when hitting an object
• FAILURE with speaker: wb_speaker_is_speaking should return true when TTS is running.