Add basic FormToggle tetsts.

Adds basic FormToggle component tests. Covers cases where the tag name
or other properties are conditionally rendered. Related to progress on
#641.

This introduces Enzyme and the react-test-renderer library as dev
dependencies.

**Testing Instructions**
Run npm i && npm run test-unit ensure tests pass. Change Dashicon logic
to ensure tests fail as they should.

components/form-toggle/test/index.js

@@ -0,0 +1,60 @@
+/**
+ * External dependencies
+ */
+import { expect } from 'chai';
+import { shallow } from 'enzyme';
+import { noop } from 'lodash';
+
+/**
+ * Internal dependencies
+ */
+import FormToggle from '../';
+
+describe( 'FormToggle', () => {
+	describe( 'basic rendering', () => {
+		it( 'without modifiers', () => {
+			const formToggle = shallow( <FormToggle /> );
+			expect( formToggle.hasClass( 'components-form-toggle' ) ).to.be.true();
+			expect( formToggle.hasClass( 'is-checked' ) ).to.be.false();
+			expect( formToggle.type() ).to.equal( 'span' );
+			expect( formToggle.find( '.components-form-toggle__input' ).prop( 'value' ) ).to.be.undefined();
+			expect( formToggle.find( '.components-form-toggle__hint' ).text() ).to.equal( 'Off' );
+			expect( formToggle.find( '.components-form-toggle__hint' ).prop( 'aria-hidden' ) ).to.be.true();
+		} );
+
+		it( 'with checked modifier', () => {
+			const formToggle = shallow( <FormToggle checked /> );
+			expect( formToggle.hasClass( 'is-checked' ) ).to.be.true();
+			expect( formToggle.find( '.components-form-toggle__input' ).prop( 'value' ) ).to.be.true();
+			expect( formToggle.find( '.components-form-toggle__hint' ).text() ).to.equal( 'On' );
+		} );
+
+		it( 'with additonal className', () => {
+			const formToggle = shallow( <FormToggle className="testing" /> );
+			expect( formToggle.hasClass( 'testing' ) ).to.be.true();
+		} );
+
+		it( 'with id property', () => {
+			const formToggle = shallow( <FormToggle id="test" /> );
+			expect( formToggle.find( '.components-form-toggle__input' ).prop( 'id' ) ).to.equal( 'test' );
+		} );
+
+		it( 'with onChange handler', () => {
+			let formToggle = shallow( <FormToggle /> );
+			let checkBox = formToggle.node.props.children.find( child => 'input' === child.type && 'checkbox' === child.props.type );
+			expect( checkBox.props.onChange ).to.equal( noop );
+
+			const testFunction = ( event ) => event;
+			formToggle = shallow( <FormToggle onChange={ testFunction } /> );
+			checkBox = formToggle.node.props.children.find( child => 'input' === child.type && 'checkbox' === child.props.type );
+			expect( checkBox.props.onChange ).to.equal( testFunction );
+		} );
+
+		it( 'with showHint false', () => {
+			const formToggle = shallow( <FormToggle showHint={ false } /> );
+
+			// When showHint is not provided this element is not rendered.
+			expect( formToggle.find( '.components-form-toggle__hint' ).length ).to.equal( 0 );
+		} );
+	} );
+} );