Unfortunately as testers its something we all run the risk of, however I would argue, that if you have taken every reasonable precaution, before carrying out the test, it cannot be your fault, but an accident.
If you can be positive the oil level was correct before the test and showed no sign of contamination, its a job for the insurers, show them the emissions printout with oil temperature to prove you must have removed and replaced the dipstick and leave it up to them.
That's what I would do
Andy