Change test/test_ssm to use generated key/crt
Change test/test_ssm to use generated key/crt

- the hardcoded certifcate needed changing anyway, as it had
  expired and we now check for that and raise an exception
  if the cert has expired.

tofu-rocketry/ssm,apel/ssm,stfc/ssm,tofu-rocketry/ssm,stfc/ssm,apel/ssm