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

jest.config.js

@@ -0,0 +1,5 @@
+const config = {
+  testMatch: ['<rootDir>/packages/slate-react/test/**/*.{js,ts,tsx,jsx}'],
+}
+
+module.exports = config