test: add tests for shallow rendering

src/__tests__/__snapshots__/shallow.test.js.snap

@@ -0,0 +1,31 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`shallow rendering React Test Instance 1`] = `
+<TouchableOpacity
+  onPress={[Function]}
+  testID="button"
+>
+  <Text
+    testID="text-button"
+  >
+    Press me
+  </Text>
+</TouchableOpacity>
+`;
+
+exports[`shallow rendering React elements 1`] = `
+<View
+  testID="root"
+>
+  <TouchableOpacity
+    onPress={[Function]}
+    testID="button"
+  >
+    <Text
+      testID="text-button"
+    >
+      Press me
+    </Text>
+  </TouchableOpacity>
+</View>
+`;

src/__tests__/shallow.test.js

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+// @flow
+import React from 'react';
+import { View, TouchableOpacity, Text } from '../__mocks__/reactNativeMock';
+import { shallow, render } from '..';
+
+const OnPressComponent = ({ onPress }) => (
+  <View testID="root">
+    <TouchableOpacity onPress={onPress} testID="button">
+      <Text testID="text-button">Press me</Text>
+    </TouchableOpacity>
+  </View>
+);
+
+test('shallow rendering React elements', () => {
+  const { output } = shallow(<OnPressComponent onPress={jest.fn} />);
+
+  expect(output).toMatchSnapshot();
+});
+
+test('shallow rendering React Test Instance', () => {
+  const { getByTestId } = render(<OnPressComponent onPress={jest.fn} />);
+  const { output } = shallow(getByTestId('root'));
+
+  expect(output).toMatchSnapshot();
+});