memblock_tests: move variable declarations to single block

Move variable declarations to a single block at the beginning of each
testing function.

Signed-off-by: Rebecca Mckeever <remckee0@gmail.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/e61431e73977f305fdd027bca99d1dc119e96d84.1662264355.git.remckee0@gmail.com