[Fiber] Add a unit test for ReactTopLevelText (facebook#8001)

src/renderers/shared/fiber/__tests__/ReactTopLevelText-test.js

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+/**
+ * Copyright 2013-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @emails react-core
+ */
+
+'use strict';
+
+var React;
+var ReactNoop;
+
+// This is a new feature in Fiber so I put it in its own test file. It could
+// probably move to one of the other test files once it is official.
+describe('ReactTopLevelText', () => {
+  beforeEach(() => {
+    React = require('React');
+    ReactNoop = require('ReactNoop');
+  });
+
+  it('should render a component returning strings directly from render', () => {
+    const Text = ({value}) => value;
+    ReactNoop.render(<Text value="foo" />);
+    ReactNoop.flush();
+    expect(ReactNoop.root.children).toEqual([{text: 'foo'}]);
+  });
+
+  it('should render a component returning numbers directly from render', () => {
+    const Text = ({value}) => value;
+    ReactNoop.render(<Text value={10} />);
+    ReactNoop.flush();
+    expect(ReactNoop.root.children).toEqual([{text: '10'}]);
+  });
+
+});