[COREVM-76] Improve heap_mem_allocator_unittest

This branch was intended to add minor fixes to `heap_mem_allocator_unittest`, however, the result is much more. The branch encompasses the following changes:

* Renaming test classes names and typo fixes.
* Fixed logic bugs in `buddy_allocation_scheme` that prevented adjacent free blocks to be properly combined.
* Added more tests to `buddy_allocation_scheme_unittest`.
* Fixed a bug in `sequential_allocation_scheme::combine_free_blocks()` that cause errors when combining free blocks.
* Added more TODO action items.