Use run-make diff output for stable output test

Author: estebank

Diff for: tests/run-make/missing-unstable-trait-bound/missing-bound.stderr

@@ -0,0 +1,12 @@
+error[E0277]: the trait bound `T: Step` is not satisfied
+ --> missing-bound.rs:2:14
+  |
+2 |     for _ in t {}
+  |              ^ the trait `Step` is not implemented for `T`
+  |
+  = note: required for `std::ops::Range<T>` to implement `Iterator`
+  = note: required for `std::ops::Range<T>` to implement `IntoIterator`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

Diff for: tests/run-make/missing-unstable-trait-bound/rmake.rs

@@ -6,10 +6,10 @@
 // Ensure that on stable we don't suggest restricting with an unsafe trait and we continue
 // mentioning the rest of the obligation chain.
 
-use run_make_support::{rust_lib_name, rustc};
+use run_make_support::{diff, rust_lib_name, rustc};
 
 fn main() {
-    rustc()
+    let out = rustc()
         .env("RUSTC_BOOTSTRAP", "-1")
         .input("missing-bound.rs")
         .run_fail()
@@ -18,5 +18,7 @@ fn main() {
             r#"
   = note: required for `std::ops::Range<T>` to implement `Iterator`
   = note: required for `std::ops::Range<T>` to implement `IntoIterator`"#,
-        );
+        )
+        .stderr_utf8();
+    diff().expected_file("missing-bound.stderr").actual_text("(stable rustc)", &out).run()
 }