Add unit-tests to exercise core APIs in mini_allocator.c module. Associated minor code refactoring / cleanup.

[gapisback]

Productize the prototype work developed to build a unit-test exercising the page allocation interfaces in mini_allocator.c . Minor cleanup of comments, interfaces to streamline APIs in this file.

In this work, we add basic exerciser test cases which will also invoke existing mini-allocator print methods.

This work precedes the other open (in-flight) issue #530 that adds unit-test cases to exercise existing mini-allocator print methods, in other sub-systems like BTree and Trunk.

[gapisback]

Discussed with Splinter core team under review for PR #539. No great value seen in this effort; see the notes in that other PR.

Nothing further to do on this proposed enhancement.