memblock tests: Add TODO and README files

Add description of the project, its structure and how to run it.
List what is left to implement and what the known issues are.

Signed-off-by: Karolina Drobnik <karolinadrobnik@gmail.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/d5e39b9f7dcef177ebc14282727447bc21e3b38f.1646055639.git.karolinadrobnik@gmail.com