tests: fix dependency on sleep in bug-874498.t

With the introduction of http://review.gluster.org/4784, there are
delays which breaks bug-874498.t which wrongly depends on healing
to finish within 2 seconds.

Fix this by using 'EXPECT_WITHIN 60' instead of sleep 2.

Change-Id: I2716d156c977614c719665a5e1f159dabf2878b5
BUG: 874498
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4796
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>