Compiletest should detect when a `.nll.stderr` file matches the `.stderr` file

[oli-obk]

This happens when NLL has been fixed up to the point where it reproduces borrowck's output. (example: #51186)

If both are the same, automatically remove the .nll.stderr in --bless mode and emit an error without --bless.

Here's what --bless does when it detects that the output of files has changed. Something similar can be done as a new check for equality of the .nll.* and .* files. It would probably be best to extract the replacing code from the current --bless impl to a separate function so it can be reused.

[FelixMcFelix]

I'd be happy to pick this up, I'll give it a go.