btrfs: tests: add self tests for extent buffer memory operations

The new self tests would populate a memory range with random bytes, then
copy it to the extent buffer, so that we can verify if the extent buffer
memory operation and memmove()/memcopy() are resulting the same
contents.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>