ReadableLaravelTests.php

<?php

class PostsTest extends TestCase {
public function testShow()
  {	
    $testpost = ['text' => 'Test post', 'id' => 1];  
    $this->instanceMock('PostRepositoryInterface')->shouldReceive('find')->with(1)->once()->andReturn($testpost);
  }
}

TestCase.php

<?php
use Mockery as m;
class TestCase extends Illuminate\Foundation\Testing\TestCase {

  //Other test case functions removed
  //$mockargs can be a string or an array of arguments to pass into m::mock()
  public function instanceMock($mockargs, $instanceName=null)
  {
    if(is_array( $mockargs))
    {
      $mockable = $mockargs[0];
      $mock = call_user_func_array('m::mock', $mockargs);		
    }
    else
    {
      $mockable = $mockargs;
      $mock = m::mock($mockable);
    }

    $instanceName = $instanceName ?: $mockable;
    App::instance($instanceName, $mock);
    return $mock;
  }
}

Perhaps if the arguments were passed as an array in a second argument?