Update test to check subnet change

This test was added to verify that changing the subnet
of a port works, and ensure no regression of bug 1533570.

Currently it tests that a port can be updated without
fixed_ip when this was previously set, but the subnet
remain the same. This patch updates the test to verify
change from subnet_a to subnet_b.

Related-Bug: #1533570
Change-Id: Ic4bbbcc3cee329a4de8f0eac710128175c4fc397
1 file changed