move esnext tests into their own fixtures folder

test/fixtures/esnext/es6-spread/simple-function-call.js

@@ -0,0 +1,6 @@
+function sum(...numbers) {
+  return numbers.reduce(function(sum, n) { return n + sum; }, 0);
+}
+
+var numbers = [1, 2, 3];
+assert.equal(sum(...numbers), 6);