move all plugin tests out of babel-core and into their appropriate folders

packages/babel-plugin-transform-jscript/test/fixtures/jscript/arrow-function/exec.js

@@ -0,0 +1,9 @@
+class MyClass {
+  test() {
+    const that = this;
+    const func = () => this === that;
+    return func();
+  }
+}
+
+assert(new MyClass().test());