Laravel 5.3: UnitTest on API - multiple function lead to error - php

Question

Hope someone can help me out here. I bootstrapped a Laravel 5.3 application and I'm writing JUnitTest in order to verifiy my API.
However, as long as I create only one function in the Test Class everything works fine:
class MobileAppTest extends TestCase {
function __construct()
{
parent::setUp();
}
public function testUserLogin()
{
$this->json('POST', '/mobile/auth', ['username' => 'empty', 'password' => 'demo'])
->seeJson([
'success' => true
]);
$this->json('POST', '/mobile/auth', ['username' => 'wrongUser', 'password' => 'demo'])
->seeJson([
'success' => false,
'state' => 2
]);
}}
When I start to add another function like this:
class MobileAppTest extends TestCase{
function __construct()
{
parent::setUp();
}
public function testUserLogin()
{
$this->json('POST', '/mobile/auth', ['username' => 'empty', 'password' => 'demo'])
->seeJson([
'success' => true
]);
$this->json('POST', '/mobile/auth', ['username' => 'wrongUser', 'password' => 'demo'])
->seeJson([
'success' => false,
'state' => 2
]);
}
public function testSecond()
{
}}
Suddenly the parameters of the HTTP POST call become empty. I have no idea why this is happening. On the server-log there is the following message poping up:
[2017-05-10 11:09:15] testing.INFO: array (
)
[2017-05-10 11:09:15] testing.ERROR: ErrorException: Undefined index: username
in app/Http/Controllers/Controller.php:36
But on the second call the parameters are present:
[2017-05-10 11:09:15] testing.INFO: array (
'username' => 'wrongUser',
'password' => 'demo',
)
Thanks for helping me out here.