Constant GRAV_USER_INSTANCE already defined

[Regaez]

When trying to write Unit Tests for my plugin, following the Codeception/_bootstrap.php approach like is done in the Grav repo, if I try and get a user before every test:

protected function _before()
{
    $grav = Fixtures::get('grav');
    $this->grav = $grav();
    $this->user = $this->grav['accounts']->load('testuser');
}

Then it throws an error:

Constant GRAV_USER_INSTANCE already defined

I did a little digging, and it seems that the AccountsServiceProvider.php is blindly defining a constant and not checking if it first exists.

This is fine when your PHP process instantiates Grav once, but when trying to write unit tests and reinstantiating Grav multiple times, it means only the first test will pass and all the rest will throw an error due to $this->grav['accounts']->load in the _before() function trying to redefine a constant.

I believe a simple fix would be wrapping them in a check within AccountsServiceProvider.php like so:

if (!defined('GRAV_USER_INSTANCE')) {
    define('GRAV_USER_INSTANCE', 'DATA');
}