Rename test fixtures using module syntax to .mjs files.

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

@@ -0,0 +1,9 @@
+export var foo = 1;
+export var foo2 = function () {};
+export var foo3;
+export let foo4 = 2;
+export let foo5;
+export const foo6 = 3;
+export function foo7 () {}
+export class foo8 {}
+foo3 = 5;