Warn when given unsupported product name. (#5845)

* Warn when given unsupported product name.

Fixes #5844.

This change means when a user launches Puppeteer with a product name
that is not supported (which at the time of this commit means it's not
`firefox` or `chrome) we will warn them about it.

Decided on just a warning vs an error because the current behaviour is
that we fallback to launching Chrome and I don't think this warrants a
breaking change.

test/mocha-utils.js

@@ -18,6 +18,7 @@ const { TestServer } = require('../utils/testserver/index');
 const path = require('path');
 const fs = require('fs');
 const os = require('os');
+const sinon = require('sinon');
 const puppeteer = require('../');
 const utils = require('./utils');
 const { trackCoverage } = require('./coverage-utils');
@@ -169,3 +170,7 @@ after(async () => {
   await state.httpsServer.stop();
   state.httpsServer = null;
 });
+
+afterEach(() => {
+  sinon.restore();
+});