Safety: assert that we use the steering req bit

[jnewb1]

TODO:

• verify all of these and split into PR for each
• VW needs to control this bit for fault avoidance, any others?

#1621

[jnewb1]

just missing vw now

[sshane]

example failure catching a safety mode not setting steer_req

======================================================================
FAIL: test_steer_req_bit (__main__.TestGmCameraSafety.test_steer_req_bit)
Asserts all torque safety modes check the steering request bit
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/batman/openpilot/panda/tests/safety/common.py", line 300, in test_steer_req_bit
    self.assertFalse(self._tx(self._torque_cmd_msg(self.MAX_TORQUE, 0)))
AssertionError: 1 is not false

[sshane]

Changed the test to not modify the safety code. It's better not to add a variable that's not used anywhere in the safety, it's more prone to bugs (was missing from the init/reset function).