[11.x] Allows deleting CreatesApplication trait (#49554)

* Improves application builder

* Uses composer instead of `debug_backtrace`

* Removes non used import

* Allows `CreateApplication` to be optional

* Fixes non-bootstrapping app

* Fixes coding style

Author: nunomaduro

src/Illuminate/Foundation/Testing/TestCase.php

@@ -4,7 +4,9 @@
 
 use Carbon\CarbonImmutable;
 use Illuminate\Console\Application as Artisan;
+use Illuminate\Contracts\Console\Kernel;
 use Illuminate\Database\Eloquent\Model;
+use Illuminate\Foundation\Application;
 use Illuminate\Foundation\Bootstrap\HandleExceptions;
 use Illuminate\Foundation\Http\Middleware\ConvertEmptyStringsToNull;
 use Illuminate\Foundation\Http\Middleware\TrimStrings;
@@ -71,11 +73,16 @@ abstract class TestCase extends BaseTestCase
     /**
      * Creates the application.
      *
-     * Needs to be implemented by subclasses.
-     *
-     * @return \Symfony\Component\HttpKernel\HttpKernelInterface
+     * @return \Illuminate\Foundation\Application
      */
-    abstract public function createApplication();
+    public function createApplication()
+    {
+        $app = require Application::inferBaseDirectory().'/bootstrap/app.php';
+
+        $app->make(Kernel::class)->bootstrap();
+
+        return $app;
+    }
 
     /**
      * Setup the test environment.

src/Illuminate/Testing/Concerns/RunsInParallel.php

@@ -3,6 +3,7 @@
 namespace Illuminate\Testing\Concerns;
 
 use Illuminate\Contracts\Console\Kernel;
+use Illuminate\Foundation\Application;
 use Illuminate\Support\Facades\ParallelTesting;
 use Illuminate\Testing\ParallelConsoleOutput;
 use RuntimeException;
@@ -176,8 +177,7 @@ protected function createApplication()
                 };
 
                 return $applicationCreator->createApplication();
-            } elseif (file_exists($path = getcwd().'/bootstrap/app.php') ||
-                      file_exists($path = getcwd().'/.laravel/app.php')) {
+            } elseif (file_exists($path = (Application::inferBaseDirectory().'/bootstrap/app.php'))) {
                 $app = require $path;
 
                 $app->make(Kernel::class)->bootstrap();