Rename test fixtures using module syntax to .mjs files.

packages/babel-plugin-transform-modules-systemjs/test/fixtures/systemjs/overview/input.mjs

@@ -0,0 +1,12 @@
+import "foo";
+import "foo-bar";
+import "./directory/foo-bar";
+import foo from "foo";
+import * as foo2 from "foo";
+import {bar} from "foo";
+import {foo as bar2} from "foo";
+
+export {test};
+export var test2 = 5;
+
+export default test;