`clippy::main_recursion` tests seem to be testing nothing anymore

[oli-obk]

I don't think this has worked at all, the other test in the same file doesn't have a .stderr file either. I'm guessing compiletest-rs didn't care that there was an extraneous file lying around if the test passes without output?

Originally posted by @oli-obk in #10426 (comment)

The relevant tests are https://github.com/rust-lang/rust-clippy/blob/master/tests/ui/crate_level_checks/no_std_main_recursion.rs and https://github.com/rust-lang/rust-clippy/blob/master/tests/ui/crate_level_checks/entrypoint_recursion.rs

[nk9]

@rustbot label L-tests "good first issue"