They do not get specially prepared cameras to test. Most of them rent, borrow or purchase their cameras and lenses.
Some of the big companies like DPR get pre-production models, but their published testing is done on a production model.
For the most part, the test results are very close from site to site. However, when sites try to hang a score on the camera, that can be misleading. A camera with a high score might not be the best for your use, the devil is in the details.
Most of the online reviewers state where they get their cameras, but it is not always obvious. I recall DPR thanking Glaziers Camera in downtown Seattle for loaning them some, and they also purchase the cameras that they want to keep around and have for further testing. Michael Reichmann gets pre-production Sony cameras, and buys or gets loaners from a Toronto Camera Store, I purchase all of my test cameras, no one would give me one
Many of the testers have had to exchange or return cameras, or wait for a firmware upgrade. There is always personal bias (also referred to as Real World Tests). Its human nature.
As for print media, they are dependent on advertising for their income, so there is a lot of pressure for them to rate the big dollar advertisers high.