All tests now check for backends they do _not_ support, not for which ones they do support. They note why backends get exluded.

./run -b ndb all" accordingly runs the following tests which it didn't before. If any of that was intentional, ndb needs to be re-excluded. test006-acls test018-syncreplication-persist test019-syncreplication-cascade test023-refint test033-glue-syncrepl test040-subtree-rename test041-aci test043-delta-syncrepl test045-syncreplication-proxied test054-syncreplication-parallel-load test055-valregex test057-memberof-refint test058-syncrepl-asymmetric

Beyond that, I seem to have confused this ITS with an "add support for back-ldif and back-null" ITS, so I'll continue that way.

back-ldif passes the tests I can run here. back-null now fails 6 tests. Need to look closer to figure out if any failures are bugs (or doc bugs), which is kind of the point of the exercise.