Auto merge of #4100 - phansch:add_stderr_length_check, r=flip1995

Add a stderr file length check to clippy_dev

This adds a check to `clippy_dev` that enforces a maximum line count for
`stderr` files. CI will fail if the line count is exceeded. It's
currently set to `320` lines.

Ideally this would be implemented in `compiletest-rs` but there are
plans to move Rust's `compiletest` into the `compiletest-rs` repository
and I don't want to do the work in `compiletest` twice. However, I also
don't want to wait until the move is done, so I added the check to
`clippy_dev` until it makes sense to add it to compiletest-rs.

cc #2038