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

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

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

Fixes: cb1e0c1d1fad ("ext4: kunit tests for extent splitting and conversion")
Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260314075258.1317579-4-yebin@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>