os.tests: Add a kernel stub trace class and use that in unit tests

os.tests: Add a kernel stub trace class and use that in unit tests

This allows to define an event layout for unit tests instead of taking the
default one, which might not permit to test everything.

This is a first step to do exhaustive unit testing of the kernel analysis

Change-Id: I6ecd85c959bf2e4c4c853deda94a0277a1e4610d
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/67623
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-by: Hudson CI