add tests for root renders

packages/enzyme-test-suite/test/ReactWrapper-spec.jsx

@@ -82,6 +82,39 @@ describeWithDOM('mount', () => {
       expect(spy).to.have.property('callCount', 1);
     });
 
+    itIf(is('>= 16'), 'should throw when mounting Portals', () => {
+      const portal = createPortal(
+        <div className="in-portal"></div>,
+        document.body,
+      );
+
+      expect(() => mount(portal)).to.throw(
+        TypeError,
+        'ReactWrapper can only wrap valid elements',
+      );
+    });
+
+    it('should throw when mounting plain text', () => {
+      expect(() => mount('Foo')).to.throw(
+        TypeError,
+        'ReactWrapper can only wrap valid elements',
+      );
+    });
+
+    it('should mount built in components', () => {
+      expect(() => mount(<div />)).not.to.throw();
+    });
+
+    it('should mount composite components', () => {
+      class Foo extends React.Component {
+        render() {
+          return <div className="in-foo"></div>;
+        }
+      }
+
+      expect(() => mount(<Foo />)).not.to.throw();
+    });
+
     describeIf(is('>= 16.3'), 'uses the isValidElementType from the Adapter to validate the prop type of Component', () => {
       const Foo = () => null;
       const Bar = () => null;

packages/enzyme-test-suite/test/ShallowWrapper-spec.jsx

@@ -20,6 +20,7 @@ import './_helpers/setupAdapters';
 import {
   createClass,
   createContext,
+  createPortal,
   createRef,
   Fragment,
   forwardRef,
@@ -77,6 +78,39 @@ describe('shallow', () => {
       expect(wrapper.children().type()).to.equal('div');
       expect(wrapper.children().props().bam).to.equal(undefined);
     });
+
+    itIf(is('>= 16'), 'should throw when shallow rendering Portals', () => {
+      const portal = createPortal(
+        <div className="in-portal"></div>,
+        document.body,
+      );
+
+      expect(() => shallow(portal)).to.throw(
+        Error,
+        'ShallowWrapper can only wrap valid elements',
+      );
+    });
+
+    it('should throw when shallow rendering plain text', () => {
+      expect(() => shallow('Foo')).to.throw(
+        Error,
+        'ShallowWrapper can only wrap valid elements',
+      );
+    });
+
+    it('should shallow render built in components', () => {
+      expect(() => shallow(<div />)).not.to.throw();
+    });
+
+    it('should shallow render composite components', () => {
+      class Foo extends React.Component {
+        render() {
+          return <div className="in-foo"></div>;
+        }
+      }
+
+      expect(() => shallow(<Foo />)).not.to.throw();
+    });
   });
 
   describe('context', () => {