I doubt there is a racetrack around large enough to achieve 300 mph. Even the big NASCAR ovals would likely tell anyone who wanted to use it to test, to Go Away. Big difference in crashes between Indy’s 230 mph and 300 MPH. Daytona and Talledega restrict NASCAR HP so they run about 200 mph.
It takes a large test track such as the ones owned by automakers or at places like the Transportation research Center in Ohio or Nardo in Italy. I think MIRA in the UK is off the list since it’s high speed test track is not large enough.