Add end to end test for KernelCpuThreadReader

Test spawns threads that do work, and checks if the times reported by
KernelCpuThreadReader correspond to the amount of work done.

Locally ran 100x to ensure that test is not flaky with given error
margins.

Test: is a test

Change-Id: Id0b45fed7871bbfad2f801480920fa2fb13b7b12