Checking in fake private keys is fine for testing. Why is it bad, out of principle, just in case you check in bad private key? I think that's a bad argument because there are a lot of benefits to being able to run end-to-end tests with some key.
Care to explain? Keeping private keys inside the repo sounds fine for me as long as these keys are only used for local development, they are rotated regularly and are only valid for localhost (in case of TLS certs).
Not GP:
If you make it normal to check in credentials and keys, then the risk of accidentally checking in prod secrets increases. It's basically making it comfortable for devs to deal with keys in repos and I think that's inherently dangerous.
You should be using automated checks to keep credentials out of your repo, not relying on individual developers. And those checks can have explicit exceptions for known safe/public/test keys, just like you might explicitly allow testing or fake credit card numbers.
