feat: Add plugin slot for login page

[ArturGaspar]

Backport of openedx#1354 to Redwood.

Description

This change adds a plugin slot for the login page allowing it to be customised.

Since there was a dependency conflict between frontend-plugin-framework and the react-hooks testing package, the react-hooks testing package has been removed and a replaced with a simple mechanism for testing hooks.

Since this touched the Login Page those have also been refactored to move away from redux connect.

Supporting information

openedx#1354

Testing instructions

1. Use env.config.jsx below.

   import { PLUGIN_OPERATIONS } from '@openedx/frontend-plugin-framework';
   
   
   const config = {
       pluginSlots: {
           login_page_slot: {
               keepDefault: true,
               plugins: [
                   {
                       op: PLUGIN_OPERATIONS.Wrap,
                       widgetId: 'default_contents',
                       wrapper: ({ component }) => {
                           return (
                               <div>
                                   <span>Hello world</span>
                                   {component}
                               </div>
                           );
                       },
                   },
               ],
           },
       },
   };
   
   export default config;
   

2. See "Hello world" text before login form

Deadline

None

Other information

Private-ref: https://tasks.opencraft.com/browse/BB-7742

[ArturGaspar]

Not necessary yet.