Fix rustfix panic on test

`run-rustfix` applies all suggestions regardless of their Applicability.
There's a flag, `rustfix-only-machine-applicable`, that does what it
says, but then the produced `.fixed` file would have invalid code from
the suggestions that weren't applied. So, I moved the cases of postfix
increment, in which case multiple suggestions are given, to the
`-notfixed` test, which does not run rustfix.

I also changed the Applicability to Unspecified since MaybeIncorrect
requires that the code be valid, even if it's incorrect.

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -1326,7 +1326,7 @@ impl<'a> Parser<'a> {
                 MultiSugg::emit_many(
                     &mut err,
                     "use `+= 1` instead",
-                    Applicability::MaybeIncorrect,
+                    Applicability::Unspecified,
                     [sugg1, sugg2].into_iter(),
                 )
             }