Custom Request object is overridden by default Illuminate\Http\Request

app.php

<?php

// ....

$app = new App\Application(
    realpath(__DIR__.'/../')
);

// ....

Application.php

<?php namespace App;


class Application extends \Laravel\Lumen\Application
{
    use Concerns\RoutesRequests; // override with our new trait
}

composer.json

{
  // ...
    "autoload": {
        "psr-4": {
            "App\\": "app/",
            "Modules\\": "Modules/",
            "Tests\\": "tests/" // <---- add this
        },
        "files": [
            "app/helpers.php"
        ]
    }
  // ...
}

MakesHttpRequests.php

<?php namespace Tests\Concerns;

use Symfony\Component\HttpFoundation\Request as SymfonyRequest;

trait MakesHttpRequests
{
    use \Laravel\Lumen\Testing\Concerns\MakesHttpRequests;
    /**
     * Call the given URI and return the Response.
     *
     * @param  string  $method
     * @param  string  $uri
     * @param  array   $parameters
     * @param  array   $cookies
     * @param  array   $files
     * @param  array   $server
     * @param  string  $content
     * @return \Illuminate\Http\Response
     */
    public function call($method, $uri, $parameters = [], $cookies = [], $files = [], $server = [], $content = null)
    {
        $this->currentUri = $this->prepareUrlForRequest($uri);

        $symfonyRequest = SymfonyRequest::create(
            $this->currentUri, $method, $parameters,
            $cookies, $files, $server, $content
        );

        return $this->response =app()->prepareResponse(
            app()->handle(app('request')::createFromBase($symfonyRequest))
        );
    }
}

RoutesRequests.php

<?php namespace App\Concerns;

use \Illuminate\Http\Request;
use Symfony\Component\HttpFoundation\Request as SymfonyRequest;

trait RoutesRequests
{
    use \Laravel\Lumen\Concerns\RoutesRequests;

    /**
     * Parse the incoming request and return the method and path info.
     *
     * @param  \Symfony\Component\HttpFoundation\Request|null  $request
     * @return array
     */
    protected function parseIncomingRequest($request)
    {
        if (! $request) {
            $request = Request::capture();
        }

        $this->instance(get_class($request), $this->prepareRequest($request));

        return [$request->getMethod(), '/'.trim($request->getPathInfo(), '/')];
    }

    /**
     * Prepare the given request instance for use with the application.
     *
     * @param  \Symfony\Component\HttpFoundation\Request $request
     * @return \Illuminate\Http\Request
     */
    public function prepareRequest(SymfonyRequest $request)
    {
        if (! $request instanceof Request) {
            $request = Request::createFromBase($request);
        }

        $request->setUserResolver(function ($guard = null) {
            return $this->make('auth')->guard($guard)->user();
        })->setRouteResolver(function () {
            return $this->currentRoute;
        });

        return $request;
    }
}

TestCase.php

<?php

use Tests\Concerns\MakesHttpRequests;
use Laravel\Lumen\Testing\DatabaseMigrations;
use Laravel\Lumen\Testing\DatabaseTransactions;

abstract class TestCase extends Laravel\Lumen\Testing\TestCase
{
    use DatabaseMigrations, // creates tables, etc.
        MakesHttpRequests; // override with our new trait
  
  // ...
}