Fix the login message so it doesn't override another

[johnbillion]

What is this PR doing?

It fixes an issue in the 0-playground.php mu-plugin which causes it to overwrite other messages on the wp-login.php screen with its own, rather than appending to them.

What problem is it solving?

If a plugin uses the login_message filter to add a message to the wp-login.php screen, 0-playground.php can overwrite it.

How is the problem addressed?

It's addressed by using add_filter() instead of the incorrect add_action(), and by appending its message instead of overwriting it.

Testing Instructions

1. Visit the Playground with the ?plugin=user-switching query variable present
2. Click the "Switch Off" link in the user profile menu in the admin toolbar
3. Visit /wp-login.php using the URL input field
4. Confirm that a "Switch back" message and link is shown by the User Switching plugin instead if being overwritten by the playground

Screenshots

Before

After

[adamziel]

This is awesome @johnbillion, thank you so much!