@TheSunIsStillShining it is not ethically possible to do a test where you cannot withdraw from the test at any time, it's very well established ethics
"Informed consent should be understood as a process, and participants have a right to withdraw at any point in the study without retribution."
cioms.ch/wp-content/uploads/2017/01/WEB-CIOMS-EthicalGuidelines.pdf
Requiring a test afterwards means that you cannot withdraw, which means it is not ethical under the current norms (and indeed the norms for at least 70 years.)
If you want to make a LAW requiring testing after such an event, you could make a law, like laws against wearing clothes or leaving a shop or entering a venue (although in actual fact none of those things actually have a law against them, there are various civil offences, and offences related to them but none are specifically illegal, but laws could be made.)
However there is no need to make such a law, the only grounds such gross invasion of forced medical procedures is wider public health, and we have the public health law to protect people here - track and trace if any cases are found everyone at the venue can be forced to isolate under the existing law. No new powers are needed and very much not forced medical procedures.
And again, the people promoting unethical shit purely because it is in their own interest increases the opposition, compulsion is not a valid strategy to compliance in any situation really.