ext4: fix mballoc-test.c is not compiled when EXT4_KUNIT_TESTS=M

ext4: fix mballoc-test.c is not compiled when EXT4_KUNIT_TESTS=M

Now, only EXT4_KUNIT_TESTS=Y testcase will be compiled in 'mballoc.c'.
To solve this issue, the ext4 test code needs to be decoupled. The ext4
test module is compiled into a separate module.

Reported-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
Closes: https://patchwork.kernel.org/project/cifs-client/patch/20260118091313.1988168-2-chenxiaosong.chenxiaosong@linux.dev/
Fixes: 7c9fa399a369 ("ext4: add first unit test for ext4_mb_new_blocks_simple in mballoc")
Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260314075258.1317579-3-yebin@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>