Add powerboard noise test
High noise has been observed for the R4/R5 powerboards. Such noisy powerboards are characterised by instable bpol 1V outputs (see Dennis' slides). @dsperlic developed a method to test if a powerboard is noisy in that manner by measuring the standard deviation of the bpol 1V.
This MR adds a new test to powertools, called PB_NOISE. It measures the bpol 1V standard deviation 100 times for loads of 0 A, 1 A and 2 A. The test passing condition is if the standard deviation is below 5. Note that in Dennis' original implementation the passing condition was 50, as the standard deviation was not divided by the sample size (which is 100, i.e. divide the passing condition by sqrt(100)=10).
Originally, this test was part of the LV_ENABLE test. I factored it out, because I think it is useful to have this as a standalone result.
This test will soon be added to the end-cap powerboard testing firmware.