Improve documentation in case of instantiating abstract test case class

[nemanja-boric-sociomantic]

If the test case Iterator fails to create an test case instance, it will show this error to the user:

turtle/src/turtle/runner/internal/Iterator.d

Lines 195 to 198 in a0da9cc

	.log.error(
	"Found an invalid test case class '{}' which doesn't " ~
	"have the default constructor defined.",
	cinfo.name

Adding the default constructor to the abstract class is pointless since the create will still fail. We could/should omit the default constructor bit if the test case is an abstract class.