Add test coverage for rootdescend verification.

Add test coverage for rootdescend verification.

Commit c1afd175, which added support for rootdescend verification to
amcheck, added only minimal regression test coverage.  Address this by
making sure that rootdescend verification is run on a multi-level index.
In passing, simplify some of the regression tests that exercise
multi-level nbtree page deletion.

Both issues spotted while rereviewing coverage of the nbtree patch
series using gcov.