I've gotten as far as getting a tag to run with it's default parameters, but I have no clue on how to fake the passing of parameters and tagdata.
I've checked the documentation for testee and munged a bit through it's own tests. I realize it is an unsupported addon as the author has moved away from ExpressionEngine, but that is why I am asking here.
Here's a working test so far:
public function test__returns_zero_with_default_attributes() {
$result = $this->_subject->modulo();
$this->assertEqual($result, 0);
}