Add test for showing remapped path in diagnostics

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

@@ -0,0 +1,5 @@
+// compile-flags: --remap-path-prefix={{src-base}}=remapped
+
+fn main() {
+    ferris //~ ERROR cannot find value `ferris` in this scope
+}

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

@@ -0,0 +1,9 @@
+error[E0425]: cannot find value `ferris` in this scope
+  --> remapped/remap-path-prefix.rs:4:5
+   |
+LL |     ferris
+   |     ^^^^^^ not found in this scope
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0425`.