Update unit test instructions in README
Update unit test instructions in README

Update the instructions for running the unit test to reflect the fact
that the tests can now be executed directly within the plugin directory
rather than requiring the plugin to be installed and tested within
horizon.

jcsp/manila-ui,openstack/manila-ui,openstack/manila-ui,openstack/manila-ui,hp-storage/manila-ui,jcsp/manila-ui,hp-storage/manila-ui,jcsp/manila-ui,hp-storage/manila-ui