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

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

@@ -0,0 +1,5 @@
+var Foo = require("Foo");
+
+function render() {
+  return <Foo />;
+}

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

@@ -0,0 +1,7 @@
+var Foo = require("Foo");
+
+var _ref = <Foo />;
+
+function render() {
+  return _ref;
+}