Improve tests

[pierfabre]

• Migrate tests to pytest when no input from user required
  • control frequency
  • has motor stopped
• Better coverage
• find how to ship tests with the python package on pypi??
• in interactive robot selftest, individual tests should be able to return a message to be displayed next to OK/FAIL
  • currently we only show the error message when it fails
  • but when we e.g test the control frequency, we'd like to see the max control frequency in the test report
• test the "has pendulum fallen" function, have the user reset the pendulum and drop it and check that we detect it
  • we often this function has a safety backstop so should be tested