Need tests for new <interleave> #2703
Description
Following up from #2538, we need tests for <interleave>. We couldn't do that right away, though, since "the test procedure validates ODDs against p5odds, which is just wrong. (Note that p5odds, the schema against which the Guidelines themselves, i.e. p5.xml & p5subset.xml are supposed to be valid, does not allow <interleave>, precisely because we do not want to use it in P5. But we do want to be able to test it. So this will take some updating of the test Makefile, too.)"