enhancement request: allow operators to support comparisons values as xpath searches

[jnpr-bowen]

Currently test operators like in-range require hard coding values for the range test. Would like to see those values be able to be expressed as xpath searches within the existing document. An example of what I would like to be able to express:

optics_check:
  - rpc: get-interface-optics-diagnostics-information
  - iterate:
      xpath: '/interface-information/physical-interface/optics-diagnostics'
      id: '../name'
      tests:
        - in-range: ./laser-rx-optical-power, ./laser-rx-power-low-warn-threshold, ./laser-rx-power-high-warn-threshold
          info: "Good: All laser receivers within warning thresholds."
          err: "Interface {{post[../name]}} laser is receiving out of range at {{post[./laser-rx-optical-power]}}. "