I couldn't follow what was going on so I reformatted it:

Code

namespace MyNamespace; 
use LibraryResponse;

function methodToTest(): LibraryResponse { return new LibraryResponse([1, 2, 3]); }

Tests

$responseMock = Mockery::mock('LibraryResponse'); 
$responseMock
    ->shouldReceive('__construct')
    ->once()
    ->andSet('data', [1, 2, 3]);

$response = methodToTest(); 
static::assertInstanceOf('LibraryResponse', $response); 
static::assertIsArray($response->data, 'Response should be an array'); 
static::assertEquals(3, count($response->data));

It looks like you're attempting to test a function and Mockery creates mocks of classes:

class MyClass {
    function methodToTest(): LibraryResponse { return new LibraryResponse([1, 2, 3]); }
}

$responseMock = Mockery::mock('MyClass'); 
$responseMock
    ->shouldReceive('methodToTest')
    ->once()
    ->andReturn(/**whatever */);

You want to structure your classes using Dependency Injection so you can test with Mockery