test: prepare BPS tree tests for new tree flavors

New BPS tree flavors are to be introduced and tested with the existing
test suite. There're a bunch of problems though:
1. The white box test uses magic constants to performs its checks, it
   is better to use constants defined by the bps_tree.h instead.
2. The bps_tree.cc test itself is not TAP-compatible, fix this by
   introducing more assertions.
3. The bps_tree_iteartor.c test is not TAP-compatible too, is uses
   the result file to check some cases. Let's remove the manual
   printing tests and modify the automated ones to cover the removed
   cases.

By the way performed minor bps_tree.cc test refactoring.

NO_DOC=test update
NO_CHANGELOG=test update