Add new tests.

Author: gnzlbg

src/test/ui/rustc_private-libtest.rs

@@ -0,0 +1,4 @@
+fn main() {
+    extern crate libtest;
+    use libtest::*; //~ ERROR unresolved import
+}

src/test/ui/rustc_private-libtest.stderr

@@ -0,0 +1,9 @@
+error[E0432]: unresolved import `libtest`
+  --> $DIR/rustc_private-libtest.rs:3:9
+   |
+LL |     use libtest::*;
+   |         ^^^^^^^ maybe a missing `extern crate libtest;`?
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0432`.