Print the checksum of the trace file on failure

When the flicker tests fail we log as well what is the checksum of the trace file which was analyzed to ensure that the file which was tested is the same available for debugging. Also, print the checksum information on the test failure message.

Test: atest FlickerTests, then check if the resulting folder contains a ".sha256" file for each WM/SF trace.
Change-Id: I448bf373e3b837c2e40317809d511a0f5236de1f