mtd: tests: don't print error messages when out-of-memory

These strings are now unnecessary and discouraged in the kernel. The
kernel will have plenty of big scary messages if kmalloc fails. These
now only serve to bloat the module.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>