What is my duty towards a Christian employer who fabricates medical test results without conducting them, and do I sin by remaining in this job?

What the laboratory owner does by not conducting the analysis and recording previous results is cheating, lying, and wrongfully consuming wealth, which is unanimously forbidden. It is not permissible for a Muslim to participate in these forbidden acts in any way. You must leave this job, because you will not be safe from participating in the forbidden, and your silence about this act may lead to harming patients.