Rename test fixtures using module syntax to .mjs files.

packages/babel-plugin-transform-modules-commonjs/test/fixtures/interop/exports-variable/input.mjs

@@ -0,0 +1,9 @@
+export var foo = 1;
+export var foo2 = 1, bar = 2;
+export var foo3 = function () {};
+export var foo4;
+export let foo5 = 2;
+export let foo6;
+export const foo7 = 3;
+export function foo8 () {}
+export class foo9 {}