Add support for UnionNodeTest

[GuntherRademacher]

These changes add support for the UnionNodeTest grammar rule and maps it to a UnionTest instance, by also lifting the restriction that a UnionTest combines only tests with the a common node type. Instead of that common type, the UnionTest's type is now set to the union of the types of the combined tests.

This fixes all of the remaining QT4 test cases with name prefix UnionNodeTest.