Jest testing for slate-react (#4459)

* initial skeleton for jest testing

* run in serial mocha, then jest on slate-react only

* run-s without glob patterns https://github.com/yarnpkg/berry/issues/22

* don't run mocha on slate-react package

* use yarn run instead of run-s

packages/slate-react/test/index.js

@@ -9,7 +9,7 @@ describe('slate-react', () => {
   describe('Editable', () => {
     describe('decorate', () => {
       // stub out some DOM stuff to avoid crashes
-      before(() => {
+      beforeEach(() => {
         const jsdom = new JSDOM()
         global.window = jsdom.window
         global.document = jsdom.window.document