add a TODO test for uninitialized warning hash bucket fencepost error

add a TODO test for uninitialized warning hash bucket fencepost error

This test assumes that a fresh hash with one key with keys from
0 to 20 will put a key in every bucket in the hash. Had we better
hash introspection rules we could write this differently and more
correctly. Nevertheless this shows the off-by-one error in key
detection used by the uninitialized warnings logic on my box.

Next patch will untodo the test and fix the bug.