Prevent local paths into libstd from leaking into ui tests

src/test/ui/impl-trait/impl-generic-mismatch.stderr

@@ -34,7 +34,7 @@ error[E0643]: method `hash` has incompatible signature for trait
 LL |     fn hash(&self, hasher: &mut impl Hasher) {}
    |                                 ^^^^^^^^^^^ expected generic parameter, found `impl Trait`
    | 
-  ::: /home/oliver/Projects/rust/rust3/src/libcore/hash/mod.rs:185:13
+  ::: $SRC_DIR/libcore/hash/mod.rs:185:13
    |
 LL |     fn hash<H: Hasher>(&self, state: &mut H);
    |             - declaration in trait here

src/tools/compiletest/src/runtest.rs

@@ -2861,6 +2861,15 @@ impl<'test> TestCx<'test> {
 
         let mut normalized = output.replace(&parent_dir_str, "$DIR");
 
+        // Paths into the libstd/libcore
+        let src_dir = self.config.src_base.parent().unwrap().parent().unwrap();
+        let src_dir_str = if json {
+            src_dir.display().to_string().replace("\\", "\\\\")
+        } else {
+            src_dir.display().to_string()
+        };
+        normalized = normalized.replace(&src_dir_str, "$SRC_DIR");
+
         if json {
             // escaped newlines in json strings should be readable
             // in the stderr files. There's no point int being correct,