Ensure tests for setters/actions check desired result is achieved

[kierdavis]

Many of our tests run some code but don't use any assertions to check the code behaved as expected. Essentially these tests are only checking that no exceptions are raised.

This is common among, but not limited to, tests for performing actions on an object or changing its state - we should be checking that the action had the desired effect.

Examples:

• test_servo_set_position
• test_motor_set_state
• test_backend_piezo_buzz
• test_power_board_make_safe (this doesn't check that the outputs are switched off, which is the whole point of make_safe! 😱)

[trickeydan]

Relevant: #214