The test bob.measure.test_script.test_hist_legends stuck the nightlies

Very often the test bob.measure.test_script.test_hist_legends get stuck in the nightlies build no matter the platform.

Check:

• https://gitlab.idiap.ch/bob/bob.nightlies/-/jobs/150561
• https://gitlab.idiap.ch/bob/bob.nightlies/-/jobs/150497
• https://gitlab.idiap.ch/bob/bob.nightlies/-/jobs/150312