timing.core: add testing for less used functions in the segment store

timing.core: add testing for less used functions in the segment store

Increases code coverage to ~80%. More importantly, makes the contract
automatically tested for new segment stores.

Change-Id: Id7c476a8df3254b01732a44e46227da4280274f2
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/82310
Reviewed-by: Jean-Christian Kouame <jean-christian.kouame@ericsson.com>
Tested-by: Jean-Christian Kouame <jean-christian.kouame@ericsson.com>
Reviewed-by: Hudson CI