Add test

This is done in a single test, because if you reset the Type without resetting the rest
it will result in different standard types.

tests/ResetTest.php

@@ -0,0 +1,26 @@
+<?php declare(strict_types=1);
+
+namespace GraphQL\Tests;
+
+use GraphQL\Type\Definition\Directive;
+use GraphQL\Type\Definition\Type;
+use GraphQL\Type\Introspection;
+use PHPUnit\Framework\TestCase;
+
+final class ResetTest extends TestCase
+{
+    public function testReset(): void
+    {
+        $string = Type::string();
+        $schema = Introspection::_schema();
+        $directives = Directive::getInternalDirectives();
+
+        Type::reset();
+        Introspection::reset();
+        Directive::reset();
+
+        self::assertNotSame($string, Type::string());
+        self::assertNotSame($schema, Introspection::_schema());
+        self::assertNotSame($directives, Directive::getInternalDirectives());
+    }
+}