Unit test coverage for Fake_file interfaces

The modified test case provides the complete coverage over
Fake_file class. These interfaces are to be used for directory
type of the objects.

BUG:987841

https://bugzilla.redhat.com/show_bug.cgi?id=987841

Change-Id: I2401423d2013ce7d90c454c72fbb60f4bc05ceb1
Signed-off-by: Chetan Risbud <crisbud@redhat.com>
Reviewed-on: http://review.gluster.org/6037
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>