generic: add a test to check move in mountpoints of the same export

generic: add a test to check move in mountpoints of the same export

Add a new test to ckeck file move (rename) operation among
different mount points which are mounting to a same export.

This should be a simple test but it recently unveils an ancient
nfsd bug. Thus let's make it to be a regresstion check.

Signed-off-by: Yongcheng Yang <yoyang@redhat.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>