Merge componentWillMount test from master (#3811)

Cherry-picked from:

Invoke setState callbacks setup in componentWillMount (#3806)

test/browser/lifecycles/componentWillMount.test.js

@@ -1,4 +1,5 @@
 import { createElement, render, Component } from 'preact';
+import { setupRerender } from 'preact/test-utils';
 import { setupScratch, teardown } from '../../_util/helpers';
 
 /** @jsx createElement */
@@ -7,8 +8,12 @@ describe('Lifecycle methods', () => {
 	/** @type {HTMLDivElement} */
 	let scratch;
 
+	/** @type {() => void} */
+	let rerender;
+
 	beforeEach(() => {
 		scratch = setupScratch();
+		rerender = setupRerender();
 	});
 
 	afterEach(() => {
@@ -39,5 +44,40 @@ describe('Lifecycle methods', () => {
 
 			expect(componentState).to.deep.equal({ value: 1 });
 		});
+
+		it('should invoke setState callbacks when setState is called in componentWillMount', () => {
+			let componentState;
+			let callback = sinon.spy();
+
+			class Foo extends Component {
+				constructor(props) {
+					super(props);
+					this.state = {
+						value: 0
+					};
+				}
+				componentWillMount() {
+					this.setState({ value: 1 }, callback);
+					this.setState({ value: 2 }, () => {
+						callback();
+						this.setState({ value: 3 }, callback);
+					});
+				}
+				render() {
+					componentState = this.state;
+					return <div />;
+				}
+			}
+
+			render(<Foo />, scratch);
+
+			expect(componentState).to.deep.equal({ value: 2 });
+			expect(callback).to.have.been.calledTwice;
+
+			rerender();
+
+			expect(componentState).to.deep.equal({ value: 3 });
+			expect(callback).to.have.been.calledThrice;
+		});
 	});
 });