Updated stderr test files.

Author: Alexander Regueiro

src/test/ui/feature-gates/feature-gate-self_in_typedefs.stderr

@@ -3,6 +3,8 @@ error[E0411]: cannot find type `Self` in this scope
    |
 LL |     Cons(T, &'a Self)
    |                 ^^^^ `Self` is only available in traits and impls
+   |
+   = help: add #![feature(self_in_typedefs)] to the crate attributes to enable
 
 error: aborting due to previous error
 

src/test/ui/inference/inference_unstable.stderr

@@ -8,5 +8,5 @@ LL |     assert_eq!('x'.ipu_flatten(), 1);
    = warning: once this method is added to the standard library, the ambiguity may cause an error or change in behavior!
    = note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
    = help: call with fully qualified syntax `inference_unstable_itertools::IpuItertools::ipu_flatten(...)` to keep using the current method
-   = note: add #![feature(ipu_flatten)] to the crate attributes to enable `inference_unstable_iterator::IpuIterator::ipu_flatten`
+   = help: add #![feature(ipu_flatten)] to the crate attributes to enable `inference_unstable_iterator::IpuIterator::ipu_flatten`