OS: R58 / R59

Tx power control in tablet/clamshell mode seems to be adopted in R58
and R59 by only testing that the values are set and read correctly.
Wifi testing for impact of the tx power control, if any, on functionality 
and performance is missing. 
This is a tracking bug to check wifi_matfunc and wifi_perf with both
the old and new SAR approaches and flag any issues. 

See internal bug numbers for context: 
38298322 (Update tx power table to meet perf numbers, R58 only)
35582729 ( Conan-SKL-Y, Chromebook StonePeak2-D1 Tx Power Control )
36080160 (Caroline, Write wifi_sarN in RO_VPD.)