exchange/tests/Feature/ChinaBankSlipTest.php

<?php

namespace Tests\Feature;

use Tests\TestCase;
use Illuminate\Foundation\Testing\WithFaker;
use Illuminate\Foundation\Testing\RefreshDatabase;
use Illuminate\Support\Facades\Storage;
use Illuminate\Http\UploadedFile;
use Artisan;

class ChinaBankSlipTest extends TestCase
{
    /**
     * A basic test example.
     *
     * @return void
     */
    public function testStore(){
    	Storage::fake('file');
    	$size_in_kb = 3072;    // 3072KB = 3MB
    	$china_bank_slips_path = UploadedFile::fake()->image('comp.jpg')->size($size_in_kb);
        
    	$response = 
		$this->postJson('api/upload-china-bankslip',[
			'china_bank_slips' => $china_bank_slips_path,
			'actual_transfer_amount' => '12',
			'details' => "123123",
			'date' => "123123"
		]);

		if(!file_exists($china_bank_slips_path) || $size_in_kb > 3072){
		    $response->assertStatus(400);    
		}
		else{
		    $response->assertSuccessful();
		}
    }
}