Add Placeholder component tests

Adds basic Placeholder component tests. Related to progress on #641.
Adds an extra conditional to make testing easier to prevent an element
that eventually renders to null from sticking in the tree when the icon
prop is not present.

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

components/placeholder/index.js

@@ -15,7 +15,7 @@ function Placeholder( { icon, children, label, instructions, className, ...addit
 	return (
 		<div { ...additionalProps } aria-label={ label } className={ classes }>
 			<div className="components-placeholder__label">
-				<Dashicon icon={ icon } />
+				{ !! icon && <Dashicon icon={ icon } /> }
 				{ label }
 			</div>
 			{ !! instructions && <div className="components-placeholder__instructions">{ instructions }</div> }

components/placeholder/test/index.js

@@ -0,0 +1,77 @@
+/**
+ * External dependencies
+ */
+import { expect } from 'chai';
+import { shallow } from 'enzyme';
+
+/**
+ * Internal dependencies
+ */
+import Placeholder from '../';
+
+describe( 'Placeholder', () => {
+	describe( 'basic rendering', () => {
+		it( 'should by default render label section and fieldset.', () => {
+			const placeholder = shallow( <Placeholder /> );
+			const placeholderLabel = placeholder.find( '.components-placeholder__label' );
+			const placeholderInstructions = placeholder.find( '.components-placeholder__instructions' );
+			const placeholderFieldset = placeholder.find( '.components-placeholder__fieldset' );
+
+			expect( placeholder.hasClass( 'components-placeholder' ) ).to.be.true();
+			// Test for empty label.
+			expect( placeholderLabel.exists() ).to.be.true();
+			expect( placeholderLabel.find( 'Dashicon' ).exists() ).to.be.false();
+			// Test for non existant instructions.
+			expect( placeholderInstructions.exists() ).to.be.false();
+			// Test for empty fieldset.
+			expect( placeholderFieldset.exists() ).to.be.true();
+		} );
+
+		it( 'should render an Dashicon in the label section', () => {
+			const placeholder = shallow( <Placeholder icon="wordpress" /> );
+			const placeholderLabel = placeholder.find( '.components-placeholder__label' ).shallow();
+
+			expect( placeholderLabel.find( 'Dashicon' ).exists() ).to.be.true();
+		} );
+
+		it( 'should render a label section and add aria label', () => {
+			const label = 'WordPress';
+			const placeholder = shallow( <Placeholder label={ label } /> );
+			const placeholderLabel = placeholder.find( '.components-placeholder__label' );
+			const child = placeholderLabel.childAt( 0 );
+
+			expect( placeholder.prop( 'aria-label' ) ).to.equal( label );
+			expect( child.text() ).to.equal( label );
+		} );
+
+		it( 'should display an instructions element', () => {
+			const element = <div>Instructions</div>;
+			const placeholder = shallow( <Placeholder instructions={ element } /> );
+			const placeholderInstructions = placeholder.find( '.components-placeholder__instructions' );
+			const child = placeholderInstructions.childAt( 0 );
+
+			expect( placeholderInstructions.exists() ).to.be.true();
+			expect( child.matchesElement( element ) ).to.be.true();
+		} );
+
+		it( 'should display a fieldset from the children property', () => {
+			const element = <div>Fieldset</div>;
+			const placeholder = shallow( <Placeholder children={ element } /> );
+			const placeholderFieldset = placeholder.find( '.components-placeholder__fieldset' );
+			const child = placeholderFieldset.childAt( 0 );
+
+			expect( placeholderFieldset.exists() ).to.be.true();
+			expect( child.matchesElement( element ) ).to.be.true();
+		} );
+
+		it( 'should add an additional className to the top container', () => {
+			const placeholder = shallow( <Placeholder className="wp-placeholder" /> );
+			expect( placeholder.hasClass( 'wp-placeholder' ) ).to.be.true();
+		} );
+
+		it( 'should add additional props to the top level container', () => {
+			const placeholder = shallow( <Placeholder test="test" /> );
+			expect( placeholder.props().test ).to.equal( 'test' );
+		} );
+	} );
+} );