Rename test fixtures using module syntax to .mjs files.

packages/babel-plugin-transform-runtime/test/fixtures/runtime/full/input.mjs

@@ -0,0 +1,9 @@
+import foo, * as bar from "someModule";
+
+export const myWord = Symbol("abc");
+export function* giveWord () {
+  yield myWord;
+}
+
+foo;
+bar;