Test that unallocated gaps within arrays have zero size.

Test that unallocated gaps within arrays have zero size.

This gets to be a bit more complex than it would first seem, as Test::More's
is_deeply() gets the length of arrays using $#array syntax, and that operation
allocates structures within the array prior to 5.12. We can't use
stringification, as we need to test undefs, so roll our own minimal test
subroutine.