Add sanity check test for StringWrapper trait

tests/StringWrapperTest.php

@@ -0,0 +1,26 @@
+<?php declare(strict_types=1);
+
+namespace Aviat\Ion\Tests;
+
+use Aviat\Ion\StringWrapper;
+use Aviat\Ion\Type\StringType;
+use PHPUnit\Framework\TestCase;
+
+class StringWrapperTest extends TestCase {
+
+	protected $wrapper;
+
+	public function setUp()
+	{
+		$this->wrapper = new class {
+			use StringWrapper;
+		};
+	}
+
+	public function testString()
+	{
+		$str = $this->wrapper->string('foo');
+		$this->assertInstanceOf(StringType::class, $str);
+	}
+
+}