Beef up invariant checking and also improve diagnostics for when an inconsistent filter has been used.
Correct filter implementation, which fixes the bug!
Hoist combined filter calculation for efficiency - I will revisit this to see if the calculation of the filler sections can also use the combined filters in an upcoming commit.
Name change to reflect the fact that what I thought was an NTestCaseBuilder bug wasn't - so the corrected test is now an example.