Add support for dynamic test providers to TestDescriptor (and TestIdentifier?)

[marcphilipp]

While implementing support for #58 in #278 we discovered that it's currently not possible to have a TestDescriptor that returns false for isTest() and an empty set for getChildren() because it will get pruned by the launcher.

Thus, a TestDescriptor should have a way to signal to the launcher that it is going to produce children during the execution phase.

[sbrannen]

@marcphilipp, when you tidy up this issue, please make sure that the recent Node refactorings in the HierarchicalTestEngine still make sense -- especially concrete implementations of the isLeaf() method and how it's used within the HierarchicalTestExecutor.

[marcphilipp]

in progress