Add tests

test/unit/components/eraser-mode.test.jsx

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+/* eslint-env jest */
+import React from 'react'; // eslint-disable-line no-unused-vars
+import {shallow} from 'enzyme';
+import EraserModeComponent from '../../../src/components/eraser-mode.jsx'; // eslint-disable-line no-unused-vars
+
+describe('EraserModeComponent', () => {
+    test('triggers callback when clicked', () => {
+        const onClick = jest.fn();
+        const componentShallowWrapper = shallow(
+            <EraserModeComponent onMouseDown={onClick}/>
+        );
+        componentShallowWrapper.simulate('click');
+        expect(onClick).toHaveBeenCalled();
+    });
+});

test/unit/components/line-mode.test.jsx

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+/* eslint-env jest */
+import React from 'react'; // eslint-disable-line no-unused-vars
+import {shallow} from 'enzyme';
+import LineModeComponent from '../../../src/components/line-mode.jsx'; // eslint-disable-line no-unused-vars
+
+describe('LineModeComponent', () => {
+    test('triggers callback when clicked', () => {
+        const onClick = jest.fn();
+        const componentShallowWrapper = shallow(
+            <LineModeComponent onMouseDown={onClick}/>
+        );
+        componentShallowWrapper.simulate('click');
+        expect(onClick).toHaveBeenCalled();
+    });
+});

test/unit/line-mode-reducer.test.js

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+/* eslint-env jest */
+import lineReducer from '../../src/reducers/line-mode';
+import {changeLineWidth} from '../../src/reducers/line-mode';
+
+test('initialState', () => {
+    let defaultState;
+
+    expect(lineReducer(defaultState /* state */, {type: 'anything'} /* action */)).toBeDefined();
+    expect(lineReducer(defaultState /* state */, {type: 'anything'} /* action */).lineWidth).toBeGreaterThan(0);
+});
+
+test('changeLineWidth', () => {
+    let defaultState;
+    const newLineWidth = 8078;
+
+    expect(lineReducer(defaultState /* state */, changeLineWidth(newLineWidth) /* action */))
+        .toEqual({lineWidth: newLineWidth});
+    expect(lineReducer(2 /* state */, changeLineWidth(newLineWidth) /* action */))
+        .toEqual({lineWidth: newLineWidth});
+    expect(lineReducer(2 /* state */, changeLineWidth(-1) /* action */))
+        .toEqual({lineWidth: 1});
+});
+
+test('invalidChangeLineWidth', () => {
+    const origState = {lineWidth: 2};
+
+    expect(lineReducer(origState /* state */, changeLineWidth('invalid argument') /* action */))
+        .toBe(origState);
+    expect(lineReducer(origState /* state */, changeLineWidth() /* action */))
+        .toBe(origState);
+});