add tests for imported_main

Author: RalfJung

src/tools/miri/tests/pass/imported_main.rs

@@ -0,0 +1,8 @@
+#![feature(imported_main)]
+
+pub mod foo {
+    pub fn mymain() {
+        println!("Hello, world!");
+    }
+}
+use foo::mymain as main;

src/tools/miri/tests/pass/imported_main.stdout

@@ -0,0 +1 @@
+Hello, world!