memblock tests: Add memblock reset function

Memblock simulator needs to be able to reset memblock data structures
between different test cases. Add a function that sets all fields to
their default values.

Add a test checking if memblock is being initialized to expected values.

Signed-off-by: Karolina Drobnik <karolinadrobnik@gmail.com>
Signed-off-by: Mike Rapoport <rppt@kernel.org>
Link: https://lore.kernel.org/r/8c185aa7e0dd68c2c7e937c9a06c90ae413e240f.1643796665.git.karolinadrobnik@gmail.com