In the BLF files, the frame timestamp are stored as a delta from the "start time". The "start time" stored in the file is the first frame timestamp rounded to 3 decimals. When we calculate the timestamp delta for each frame, we were previously calculating it using the non-rounded "start time".

This new code, use the "start time" as the first frame timestamp truncated to 3 decimals.

Making sure all tests messages contains a timestamp to avoid situation where the first written message have timestamps equal to zero.