[sliceable_switch] fixed test for REST-API /networks

Fixed test(feature) for REST interface to configure networks.
In this test, all steps were done via REST API (e.g.. A created
network object was asserted with getting the object via REST API).
Now, all steps except the test target operation in a scenario
are done by accessing the slice DB directly.
This patch also includes the following:
  - added test for attachment network objects
  - changed repeated "Then" to "And"
  - fixed typos