demo.php

<?php

Mock::instance(App\Http\Requests\AuthRequest::class)->bind();
Mock::instance(Illuminate\Auth\Guard::class)->expects(function ($auth) {
    $auth->shouldReceive('attempt')->andReturn(false);
})->bind();

Mock.php

<?php

class Mock
{
    /**
     * Full path of the class name to mock.
     *
     * @var string
     */
    protected $class;

    /**
     * Mockery instance.
     *
     * @var Mockery
     */
    protected $instance;

    /**
     * Create a new mock instance.
     *
     * @param  string  $class
     * @return Mock
     */
    private function __construct($class)
    {
        $this->class = $class;
        $this->instance = Mockery::mock($class);
    }

    /**
     * Create a mock instance for the given class.
     *
     * @param  string  $class
     * @return Mock
     */
    public static function instance($class)
    {
        return new static($class);
    }

    /**
     * Add the expectations from the mock.
     *
     * @param  Closure  $callback
     * @return Mock
     */
    public function expects(Closure $callback)
    {
        $callback->__invoke($this->instance);

        return $this;
    }

    /**
     * Bind the instance to to IoC container.
     *
     * @return void
     */
    public function bind()
    {
        app()->instance($this->class, $this->instance);
    }
}