Demonstration of how to use a `resetAuth` function to fully reset auth state in-between Laravel unit tests. For theory, see: https://stackoverflow.com/a/57941133/6141025

TestCase.php

<?php

use Illuminate\Auth\SessionGuard;

// ...

/**
 * Resets AuthManager state by logging-out the user from all auth guards.
 * This is used between unit tests to wipe cached auth state.
 *
 * @param array $guards
 * @return void
 */
protected function resetAuth(array $guards = null) : void
{
    $guards = $guards ?: array_keys(config('auth.guards'));

    foreach ($guards as $guard) {
        $guard = $this->app['auth']->guard($guard);
        
        // \Log::debug("logging out guard: {$guard}");

        if ($guard instanceof SessionGuard) {
            $guard->logout();
        }
    }

    $protectedProperty = new \ReflectionProperty($this->app['auth'], 'guards');
    $protectedProperty->setAccessible(true);
    $protectedProperty->setValue($this->app['auth'], []);
}