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

packages/babel-plugin-transform-react-constant-elements/test/fixtures/constant-elements/children/actual.js

@@ -0,0 +1,7 @@
+var Foo = React.createClass({
+  render: function() {
+    return <div className={this.props.className}>
+      <span />
+    </div>;
+  }
+});

packages/babel-plugin-transform-react-constant-elements/test/fixtures/constant-elements/children/expected.js

@@ -0,0 +1,9 @@
+var _ref = <span />;
+
+var Foo = React.createClass({
+  render: function () {
+    return <div className={this.props.className}>
+      {_ref}
+    </div>;
+  }
+});