PHPUnit Test Failing - 'PHPUnit\Framework\Assert' not found

Laravel project, simple test, only tries to load the default page '/'

SIMPLE TEST

<?php

namespace Tests\Feature;

use Tests\TestCase;
use Illuminate\Foundation\Testing\WithoutMiddleware;
use Illuminate\Foundation\Testing\DatabaseMigrations;
use Illuminate\Foundation\Testing\DatabaseTransactions;

class ExampleTest extends TestCase
{
    /**
     * A basic test example.
     *
     * @return void
     */
    public function testBasicTest()
    {
        $response = $this->get('/');

        $response->assertStatus(200);
    }
}

ERROR MESSAGE via PhpStorm

Class 'PHPUnit\Framework\Assert' not found
/var/www/vhosts/app-native/app-app/vendor/laravel/framework/src/Illuminate/Foundation/Testing/TestResponse.php:52
/var/www/vhosts/app-native/app-app/tests/Feature/ExampleTest.php:21

ERROR MESSAGE via CLI

# php vendor/phpunit/phpunit/phpunit
PHPUnit 5.6.4 by Sebastian Bergmann and contributors.

EEE..                                                               5 / 5 (100%)

Time: 1.78 seconds, Memory: 16.00MB

There were 3 errors:

1) Tests\Feature\ExampleTest::testBasicTest
Error: Class 'PHPUnit\Framework\Assert' not found

/var/www/vhosts/app-native/app-app/vendor/laravel/framework/src/Illuminate/Foundation/Testing/TestResponse.php:52
/var/www/vhosts/app-native/app-app/tests/Feature/ExampleTest.php:21

2) Tests\Feature\JsonTest::testSignup
Error: Class 'PHPUnit\Framework\Assert' not found

/var/www/vhosts/app-native/app-app/vendor/laravel/framework/src/Illuminate/Foundation/Testing/TestResponse.php:52
/var/www/vhosts/app-native/app-app/tests/Feature/JsonTest.php:29

3) Tests\Feature\RoutesTest::testBasicTest
Error: Class 'PHPUnit\Framework\Assert' not found

/var/www/vhosts/app-native/app-app/vendor/laravel/framework/src/Illuminate/Foundation/Testing/TestResponse.php:52
/var/www/vhosts/app-native/app-app/tests/Feature/RoutesTest.php:21

ERRORS!
Tests: 5, Assertions: 2, Errors: 3.

Upon further investigation...

The Assert class is not found via the IDE either.

Looking into the composer autoloader, the only PHPUnit\Framework classes being loaded is the "ForwardCompatibility/TestCase ???!!!

3285'PHPUnit\\Framework\\TestCase' => $vendorDir . '/phpunit/phpunit/src/ForwardCompatibility/TestCase.php',
3825'PHPUnit\\Framework\\TestCase' => __DIR__ . '/..' . '/phpunit/phpunit/src/ForwardCompatibility/TestCase.php',

Here is my composer file, for good measure...

{
    "name": "app/webapp",
    "description": "app Web App (API & Frontend).",
    "keywords": ["framework", "laravel"],
    "license": "MIT",
    "type": "project",
    "require": {
        "php": ">=5.6.4",
        "laravel/framework": "5.4.*",
        "laravel/tinker": "~1.0",
        "laravelcollective/html": "~5.0",
        "laracasts/flash": "~1.3",
        "maatwebsite/excel": "~2.1",
        "guzzlehttp/guzzle": "~6.2",
        "doctrine/dbal": "~2.5",
        "laravel/cashier": "~7.0",
        "league/flysystem-aws-s3-v3": "~1.0",
        "zizaco/entrust": "1.7.0",
        "barryvdh/laravel-ide-helper": "^2.2",
        "blueimp/jquery-file-upload": "^9.14",
        "ipunkt/laravel-analytics": "^1.3",
        "braintree/braintree_php": "^3.21",
        "tymon/jwt-auth": "0.5.*",
        "f2m2/apidocs": "~2.0",
        "barryvdh/laravel-cors": "0.8.*",
        "pulkitjalan/geoip": "~2.4",
        "aws/aws-sdk-php-laravel": "^3.1",
        "vsmoraes/laravel-pdf": "^1.0",
        "propaganistas/laravel-phone": "^2.8",
        "activecampaign/api-php": "~2.0"
    },
    "require-dev": {
        "fzaninotto/faker": "~1.4",
        "mockery/mockery": "0.9.*",
        "phpunit/phpunit": "~5",
        "phpspec/phpspec": "~2.1",
        "ozankurt/repoist": "^1.0",
        "symfony/dom-crawler": "~3.1",
        "symfony/css-selector": "~3.1"
    },
    "autoload": {
        "classmap": [
            "database"
        ],
        "psr-4": {
            "App\\": "app/"
        }
    },
    "autoload-dev": {
        "psr-4": {
            "Tests\\": "tests/"
        }
    },
    "scripts": {
        "post-root-package-install": [
            "php -r \"file_exists('.env') || copy('.env.example', '.env');\""
        ],
        "post-create-project-cmd": [
            "php artisan key:generate"
        ],
        "post-install-cmd": [
            "Illuminate\\Foundation\\ComposerScripts::postInstall",
            "php artisan ide-helper:generate",
            "php artisan ide-helper:meta",
            "php artisan optimize"
        ],
        "post-update-cmd": [
            "Illuminate\\Foundation\\ComposerScripts::postUpdate",
            "php artisan ide-helper:generate",
            "php artisan ide-helper:meta",
            "php artisan optimize"
        ]
    },
    "config": {
        "preferred-install": "dist",
        "sort-packages": true
    }
}