Add a couple more tests

Author: petrochenkov

src/test/ui/imports/extern-prelude-extern-crate-restricted-shadowing.rs

@@ -14,4 +14,13 @@ mod m {
     }
 }
 
+macro_rules! define_other_core {
+    () => {
+        extern crate std as core;
+        //~^ ERROR macro-expanded `extern crate` items cannot shadow names passed with `--extern`
+    }
+}
+
+define_other_core!();
+
 fn main() {}

src/test/ui/imports/extern-prelude-extern-crate-restricted-shadowing.stderr

@@ -1,3 +1,12 @@
+error: macro-expanded `extern crate` items cannot shadow names passed with `--extern`
+  --> $DIR/extern-prelude-extern-crate-restricted-shadowing.rs:19:9
+   |
+LL |         extern crate std as core;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL | define_other_core!();
+   | --------------------- in this macro invocation
+
 error[E0659]: `Vec` is ambiguous (macro-expanded name vs less macro-expanded name from outer scope during import/macro resolution)
   --> $DIR/extern-prelude-extern-crate-restricted-shadowing.rs:13:9
    |
@@ -14,6 +23,6 @@ LL |         extern crate std as Vec;
 LL | define_vec!();
    | -------------- in this macro invocation
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0659`.

src/test/ui/imports/extern-prelude-extern-crate-shadowing.rs

@@ -0,0 +1,12 @@
+// compile-pass
+// aux-build:two_macros.rs
+
+extern crate two_macros as core;
+
+mod m {
+    fn check() {
+        core::m!(); // OK
+    }
+}
+
+fn main() {}