ctf: better test event headers (bug 446190)

This fixes a problem with non 8 bit aligned event headers. Before
the alignemnt was assumed to be 8 bits, now it is checked.

Change-Id: Ib938755d6191bbe8fea34400f5e5183cb2dd7035
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/36168
Tested-by: Hudson CI
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>