adding test plan for PMON enhancements for chassis (#2695)

What is the motivation for this PR?
adding test plan for PMON enhancements for chassis

How did you do it?
Based on the code changes in the associated PRs:
Configure and show for platform chassis_modules #1145
CHASSIS_STATE_DB on control-card for chassis state #395
PSUd changes to compute power-budget for Modular chassis #104
Introduce APIs for modular chassis support #124
Common power consumption and supply APIs for modular chassis #136
Thermalctld APIs for recording min and max temp #131
Modular Chassis - Midplane monitoring APIs #148
Modular-Chassis: Show midplane status #1267