Move tests from obsolete test files to test.lisp and adapt them to the

current Rucksack version.  Start testing btrees: the basics work, but
with large btrees (20,000 nodes or more?) I get GC errors again.  It
seems that blocks are deallocated that shouldn be, so my guess is that
these are due to a mismatch between the liveness of objects that are
on disk and their corresponding in-memory versions.