selftests: filesystems: Add functional test for the abort file in fusectl

This patch add a simple functional test for the "abort" file
in fusectlfs (/sys/fs/fuse/connections/ID/abort).

A simple fuse daemon is added for testing.

Related discussion can be found in the link below.

Link: https://lore.kernel.org/all/CAOQ4uxjKFXOKQxPpxtS6G_nR0tpw95w0GiO68UcWg_OBhmSY=Q@mail.gmail.com/


Signed-off-by: Chen Linxuan <chenlinxuan@uniontech.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>