Why don’t aircraft undergo contaminated-runway landing-distance and rejected-takeoff certification testing?
Baseline performance data was developed some years ago by an FAA test program called the Joint Winter Runway Friction Program to develop mathematical models of the effects of the different types of surface contamination.  Manufacturers apply the models to develop their own aircraft's performance correction factors for various contaminated runway conditions.

Otherwise, as Ron comments, the manufacturers would have a massive test burden to develop customized data for a huge range of contaminants, and would have to go find the test conditions somewhere for each, adding eleventy jillion dollars to the typical development program.  The mathematical models, along with the safety factors built into the calculations, do the job just as well.

This Advisory Circular talks about it.