Better group RFC ui tests together

tests/ui/rfcs/rfc-2005-default-binding-mode/const.stderr

@@ -0,0 +1,18 @@
+error[E0308]: mismatched types
+  --> $DIR/const.rs:14:9
+   |
+LL | const FOO: Foo = Foo{bar: 5};
+   | -------------- constant defined here
+...
+LL |     match &f {
+   |           -- this expression has type `&Foo`
+LL |         FOO => {},
+   |         ^^^
+   |         |
+   |         expected `&Foo`, found `Foo`
+   |         `FOO` is interpreted as a constant, not a new binding
+   |         help: introduce a new binding instead: `other_foo`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.