Regression test portability: basic/file-snapshot.t

- For some odd reason, setfattr(1) could fail on big-file just after
  it was created by touch(1). Add a test in between to wait for the
  file to be visible by ls(1).
- After switching to a snapshot, the file content is still in kernel
  cache, and the old content may be readen for a few seconds after
  the snapshot switch. Use ( cd $M0 && umount $M0 ) to flush vnodes
  and therefore invalidate the cache.
- Unmount the mount point at the end of the test for the cleanliness
  sake.

BUG: 1129939
Change-Id: I5a0d14db13e79323147761dfe50c41ae6a1cb844
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/9220
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>