Added test for GenericContainer implementation + implemented exec related stuff

tests/Integration/GenericContainerTest.php

@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Testcontainers\Tests\Integration;
+
+use Testcontainers\Container\GenericContainer;
+
+class GenericContainerTest extends ContainerTestCase
+{
+    public static function setUpBeforeClass(): void
+    {
+        self::$container = (new GenericContainer('alpine'))
+            ->withCommand(['tail', '-f', '/dev/null'])
+            ->start();
+    }
+
+    public function testExec(): void
+    {
+        $actual = self::$container->exec(['echo', 'testcontainers']);
+        self::assertSame('testcontainers', $actual);
+    }
+}