test: Make a dedicated testsuite for rustfix

This commit adds a dedicated mode to compiletest for running rustfix tests,
adding a new `src/test/rustfix` directory which will execute all tests as a
"rustfix" test, namely requiring that a `*.fixed` is next to the main file which
is the result of the rustfix project's application of fixes.

The `rustfix` crate is pulled in to actually perform the fixing, and the rustfix
compiletest mode will assert a few properties about the fixing:

* The expected fixed output must be the same as rustc's output suggestions
  applied to the original code.
* The fixed code must compile successfully
* The fixed code must have no further diagnostics emitted about it

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