Add test cases to exercise mini_unkeyed_print() and mini_keyed_print() methods found in mini_allocator.c

[gapisback]

Mini-allocator has couple of print methods to print contents of meta-pages of keyed and unkeyed mini-allocator. Add new test cases to invoke these print methods so that (a) we can inspect their outputs, and (b) ensure that these print methods continue to be exercised and stay healthy.

In the fix for issue #512 , a small extension was done to existing unit/btree_stress_test.c test to call btree_print_tree(). This test should be modularized for re-use to add a new test case that can invoke mini_keyed_print().

A similar extension should be done to find a way to exercise mini_unkeyed_print().

[gapisback]

The code changes to address the requirements in this issue were aired out under PR #531 and discussed with Splinter-dev team. There was no great agreement to incorporate that work. See the notes in that PR.

Nothing further to do here.