Add simple test for externalRuntime transformer

Just import a module namespace and see what happens.

test/fixtures/transformation/optional-external-runtime/basic/actual.js

@@ -0,0 +1 @@
+import * as foo from "someModule";

test/fixtures/transformation/optional-external-runtime/basic/expected.js

@@ -0,0 +1,5 @@
+"use strict";
+
+var _to5Runtime = require("6to5-runtime");
+
+var foo = _to5Runtime.interopRequireWildcard(require("someModule"));

test/fixtures/transformation/optional-external-runtime/options.json

@@ -0,0 +1,4 @@
+{
+  "runtime": "6to5-runtime",
+  "optional": ["externalRuntime"]
+}