add TowerLogo tests

__tests__/components/TowerLogo.test.jsx

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+import React from 'react';
+import { MemoryRouter } from 'react-router-dom';
+import { mount } from 'enzyme';
+import TowerLogo from '../../src/components/TowerLogo';
+
+let logoWrapper;
+let towerLogoElem;
+let brandElem;
+
+const findChildren = () => {
+  towerLogoElem = logoWrapper.find('TowerLogo');
+  brandElem = logoWrapper.find('Brand');
+};
+
+describe('<TowerLogo />', () => {
+  test('initially renders without crashing', () => {
+    logoWrapper = mount(<MemoryRouter><TowerLogo /></MemoryRouter>);
+    findChildren();
+    expect(logoWrapper.length).toBe(1);
+    expect(towerLogoElem.length).toBe(1);
+    expect(brandElem.length).toBe(1);
+  });
+
+  test('adds navigation to route history on click', () => {
+    const onLogoClick = jest.fn();
+    logoWrapper = mount(<MemoryRouter><TowerLogo onClick={onLogoClick} /></MemoryRouter>);
+    findChildren();
+    expect(towerLogoElem.props().history.length).toBe(1);
+    logoWrapper.simulate('click');
+    expect(towerLogoElem.props().history.length).toBe(2);
+  });
+
+  test('gracefully handles not being passed click handler', () => {
+    logoWrapper = mount(<MemoryRouter><TowerLogo /></MemoryRouter>);
+    findChildren();
+    expect(towerLogoElem.props().history.length).toBe(1);
+    logoWrapper.simulate('click');
+    expect(towerLogoElem.props().history.length).toBe(1);
+  });
+
+  test('handles mouse over and out state.hover changes', () => {
+    const onLogoClick = jest.fn();
+    logoWrapper = mount(<MemoryRouter><TowerLogo onClick={onLogoClick} /></MemoryRouter>);
+    findChildren();
+    findChildren();
+    expect(brandElem.props().src).toBe('tower-logo-header.svg');
+    brandElem.props().onMouseOver();
+    expect(towerLogoElem.state().hover).toBe(true);
+    logoWrapper.update();
+    findChildren();
+    expect(brandElem.props().src).toBe('tower-logo-header-hover.svg');
+    brandElem.props().onMouseOut();
+    expect(towerLogoElem.state().hover).toBe(false);
+    logoWrapper.update();
+    findChildren();
+    expect(brandElem.props().src).toBe('tower-logo-header.svg');
+  });
+});