tmf.core: Add unit tests for state system re-read

tmf.core: Add unit tests for state system re-read

This adds a unit test for a state system with full backend where the
results are re-read from a previously existing file.

Change-Id: Iab5fab6c6282f2411d2e4a0d3b0da3824653cb36
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/84036
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>