Claude sonnet 4 (this time last year) did do this. It once made simulation if a test script passing. Literally a script that just echoed test names and then said pass.
I'm sure it can happen, hence why I said to keep an eye out. Its main mode of operation is not to cook the tests however.
Our team was doing a similar task to move between test frameworks, and I had to do a git diff of hundreds of thousands of lines to try and work out where a test had disappeared to.
Your fault. You should have used a model from 0.000005 seconds ago!
What change? That you should not fake the results of a test because that defeats the whole purpose of a test has been known before there were computers.