The AssertionError exception is intended to be used when an internal
condition that must exist, does not exist.  When dealing with values
received from an outer layer, that we can't control, it's better to
use the more appropriate ValueError.

Another reason is that code such as:

    def run_avocado(self):
        self._setup_environment_variables()
        try:
            self._tag_start()
            self._run_avocado()
    ...
        except AssertionError as detail:
             self.__status = 'FAIL'
             self.__fail_class = detail.__class__.__name__
             self.__fail_reason = str(detail)
             self.__traceback = stacktrace.prepare_exc_info(sys.exc_info())
    ...

Can be afected by the AssertionErrors, and return a test failure
instead of a test error.

Reference: https://docs.python.org/3/library/exceptions.html#ValueError
Reference: https://docs.python.org/3/library/exceptions.html#AssertionErrorSigned-off-by: NCleber Rosa <crosa@redhat.com>