Consider adding a time keeping device

In mbed OS testing, we currently struggle to have a trusted time keeping source to compare the hardware timers. We have been using the host PC as the trusted time source, but it isn't a real time system so the time can skew significantly by the time it is communicated with the device.

If we could have something like an RTC or a 555 timer circuit as a trusted time source, it would allow us to more accurately measure time and compare it with the device-reported time.

Start by reviewing the mbed OS testing setup and how host-PC time is communicated to the device. Compare an RTC or 555 timer as a trusted source; done would be a specified, implemented test-shield source that enables accurate comparison with device-reported time. The issue names no files or tests, so locating the hardware and test entry points is necessary first.

Written by the indexing model from the issue text.