Create multiple .fixed files for diagnostics with multiple suggestions

[Alexendoo]

Some clippy tests disable rustfix because they produce multiple alternative suggestions e.g. join_absolute_paths. This PR allows there to be a .fixed file per alternative suggestion, in that example join_absolute_paths.1.fixed and join_absolute_paths.2.fixed

There are some lints that produce multiple suggestions that are intended to be combined e.g.

error: this binding can be a slice pattern to avoid indexing
  --> tests/ui/index_refutable_slice/if_let_slice_binding.rs:21:17
   |
LL |     if let Some(slice) = slice {
   |                 ^^^^^
   |
help: try using a slice pattern here
   |
LL |     if let Some([slice_0, ..]) = slice {
   |                 ~~~~~~~~~~~~~
help: and replace the index expressions here
   |
LL |         println!("{}", slice_0);
   |                        ~~~~~~~

These would have to use //@no-rustfix for now, but I think moving them to multipart suggestions is correct in the long run