diag-mode: return InTest if host is in diag mode

Per discussion with DMTF and clarification of InTest state within
http://redfish.dmtf.org/schemas/v1/Resource.json#/definitions/Status


it was determined that this would represent the situation where the host
is collecting diagnostic data. This maps to the new DiagnosticMode
state added to the xyz.openbmc_project.State.Host.HostState namespace.

Tested:
Verified that Redfish API returned expected result when in
DiagnosticMode:
  "Status": {
    "Health": "OK",
    "HealthRollup": "OK",
    "State": "InTest"
  },

Change-Id: I1c5deb5fdc251a5dcd4d1d01f4eedb6c507ded0a
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>