date-773271: Allow custom tests for arg values
date-773271: Allow custom tests for arg values

Sometimes we can't provide an identity equals value for an argument
check, but we still want to check the properties of the value, so we
allow 'value' to be a function which we call, passing it the actual
value, and expect the function to call assert to prove that the provided
value is correct.

Signed-off-by: Joe Walker <7a17872fbb32c7d760c9a16ee3076daead11efcf@mozilla.com>

joewalker/gcli,mozilla/gcli,mozilla/gcli,mozilla/gcli,joewalker/gcli