Small refactoring in tests development docs
Small refactoring in tests development docs

- Merged the "Writing Tests" and "Tests Structure" sections:
  both had complementary information.

- Moved "Further Testing" to the end - the intention is to remove this
  section and put details about keystone's tempest and devstack plugins

Change-Id: Ia27b094939a3c76e68ac8e87ef34b7ee611c1929

openstack/keystone,rajalokan/keystone,openstack/keystone,mahak/keystone,rajalokan/keystone,mahak/keystone,openstack/keystone,rajalokan/keystone,mahak/keystone