Test sparse ordinal behavior

[ftobia]

There is a behavior I intended which I have not tested (see also: mattias-lundell@a29a07f#commitcomment-9290806). The behavior is that you need not specify all the ordinals from the beginning or end for order to be respected. That is, if you mark a test as "third", it will be the third test to run, regardless of whether you have tests marked "first" and "second" (assuming of course that you have at least 3 tests).

There should be tests for the from-the-beginning case, the from-the-end case, and various edge cases.

[ftobia]

Judging from #29 test failures, ordinal behavior is broken.