tests: Test the open-fd count for a file on brick

Change-Id: I59bc4974eff93e92c4f7041d3722ac60e88b0734
BUG: 908146
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/4471
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>