test : Fix for spurious failure

Problem : Once the features.uss is enabled it does not wait for
the process to be created. And if we try to check for
the pid of the snapd then it will not be present which
causes a failure.

Solution : Adding a EXPECT_WITHIN which waits to get the pid
until certain time period.

Change-Id: I5fdda9beecf867b7544f2e4b830f698ddf6e3bec
BUG: 1145189
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/8809
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>