Update Spec-Tests framework to support test cases marked as invalid

[cmoesel]

Some test cases are marked as invalid. For example:

<test name="SuccessorOverflowDt">
  <expression invalid="true">successor of DateTime(9999, 12, 31, 23, 59, 59, 999)</expression>
  <!-- EXPECT: The result of the successor operation exceeds the maximum value allowed for the type -->
</test>

Since these don't have an output, our tests basically treat them like a no-op. We should consider if the correct behavior is to expect an exception (probably?) or a null result (maybe).

Some tests are marked as semantically invalid, like this one:

<test name="TimeUpperBoundHours">
  <expression invalid="semantic">@T24:59:59.999</expression>
  <!-- Translation Error: Invalid date-time input (T24:59:59.999). Use ISO 8601 date time representation (yyyy-MM-ddThh:mm:ss.mmmmZhh:mm). -->
</test>

We currently have to put these in the skip-list.txt file because the translator errors cause issues. But we should try to avoid translating these ones at all, if possible.