inline `tests/mir-opt` docs into the dev-guide

[jyn514]

https://github.com/rust-lang/rust/blob/HEAD/tests/mir-opt/README.md has quite a lot of info but it's hard to find. We should:

1. Move all its info into the guide (Add a description of how FileCheck is used for mir-opt tests #2718 is a good start)
2. Delete it and replace it with a link to the new guide docs

We should not just duplicate the docs, that'll cause them to get out of sync.