Fix remap-path-prefix UI test case.

src/test/ui/remap-path-prefix.rs

@@ -1,5 +1,9 @@
 // compile-flags: --remap-path-prefix={{src-base}}=remapped
 
 fn main() {
-    ferris //~ ERROR cannot find value `ferris` in this scope
+    // We cannot actually put an ERROR marker here because
+    // the file name in the error message is not what the
+    // test framework expects (since the filename gets remapped).
+    // We still test the expected error in the stderr file.
+    ferris
 }

src/test/ui/remap-path-prefix.stderr

@@ -1,5 +1,5 @@
 error[E0425]: cannot find value `ferris` in this scope
-  --> remapped/remap-path-prefix.rs:4:5
+  --> remapped/remap-path-prefix.rs:8:5
    |
 LL |     ferris
    |     ^^^^^^ not found in this scope