tmf: Add unit tests for CTF event contexts

tmf: Add unit tests for CTF event contexts

This required adding a new trace "kernel_vm" to the set of
downloaded traces, since we didn't have a trace with event
contexts.

Change-Id: Iee56b26b29088c3d6d017a4adcdf6e9f53bb8688
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/13991
Tested-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
IP-Clean: Matthew Khouzam <matthew.khouzam@ericsson.com>