tests/test_devicestate: Add additional tests for the device state API

This patch adds more tests that exercise the device state API. This includes:

* Tests that cover adding a device state provider, as well as deleting a
  device state provider. This also verifies that you cannot add an
  already added device state provider, and cannot delete an already
  deleted device state provider.
* A test that covers changing device state and receiving said updates
  from a device state subscriber. This also covers hitting both the
  device state cache as well as a custom device state provider.
* A test that covers converting device state to channel state and device
  state values to a string representation and back.
* A test that covers obtaining device state from an active channel and a
  channel driver that provides its own device state.

Change-Id: I2adca67ffb405cd8625a5d6df1e3f9b3d945c08d