tests: lib: uuid: add heap arena for MINIMAL_LIBC compatibility

The UUID library utilities require dynamic memory allocation for certain
operations, but MINIMAL_LIBC provides no malloc heap by default. This
causes test failures when UUID functions attempt to allocate memory.

Add CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE=256 to provide a minimal heap
arena sufficient for UUID operations which typically
require small heap allocations.

Signed-off-by: Mohamed Moawad <moawad@synopsys.com>