Allow hook into Jest setup files

[robmcguinness]

Some projects like to use https://github.com/airbnb/enzyme which requires a gloabl jest configuration:

const { configure } = require('enzyme');
const Adapter = require('enzyme-adapter-react-16');

configure({ adapter: new Adapter() });

A clean way to add this configuration is through Jest's setupFiles configuration property: https://github.com/Availity/availity-workflow/blob/master/packages/workflow-plugin-react/test.js#L18

Maybe a convention based approach could work where each workflow plugin can detect a jestSetup.js file at the root of the project and include if found:

setupFiles: ['raf/polyfill', '<rootDir>/jestSetup.js'],

[robmcguinness]

Having the flag --runInBand is recommend by VS Code to prevent creating a worker pool of child processes.

function unit() {
  const argv = process.argv.slice(2);
  const jestConfig = JSON.stringify(create());
  argv.push(`--config=${jestConfig}`);
  argv.push('--env=jsdom');
  argv.push('--runInBand');

  jest.run(argv);

  return Promise.resolve();
}

[TheSharpieOne]

Probably also want to give access to setupTestFrameworkScriptFile for react at least (angular currently does this to set up angular and angular-mocks)