Add xfailed test for #2123

Author: catamorphism

src/test/compile-fail/issue-2123.rs

@@ -0,0 +1,9 @@
+// xfail-test
+// error-pattern:unresolved import: m::f
+import x = m::f;
+
+mod m {
+}
+
+fn main() {
+}