RangeTest: move some of the test cases

Moved the type enforcement tests to the new TypeMismatchTest.
SingleAndMultiRangeNextTest: split up

Replaced SingleAndMultiRangeNextTest with
- SimpleSingleRangeNextTest
- SimpleMultiRangeNextTest
- AdvancedSingleRangeNextTest
AdvancedSingleRangeNextTest: javadoc and new tests
SimpleMultiRangeNextTest: minor tweaks
GenTestCase: add support for intervals