Allow specification of test_case/0 function return in -easy_test

[jrwest]

It should be possible to specify the return list from a test_case/0 function (as opposed to writing the function) using a new config option passed to -easy_test([...]).. The value should be identical to the list of values returned by a test_case/0 function. If given, the has_config option is considered to be true regardless of its value.