statesystem: Add utility methods to help unit test state systems

statesystem: Add utility methods to help unit test state systems

These methods and classes can be used to easily unit test the intervals
for an attribute for the duration of the tree, or the values of many attributes
at a given timestamp.

Change-Id: I3eb9fc8bdc1d2ba8a388c9e0f979b506a60e3ae1
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/66403
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Reviewed-by: Alexandre Montplaisir <alexmonthy@efficios.com>