add toBeIn

tests/Features/Expect/toBeIn.php

@@ -0,0 +1,11 @@
+<?php
+
+use PHPUnit\Framework\ExpectationFailedException;
+
+test('passes', function () {
+    expect('a')->toBeIn(['a', 'b', 'c']);
+});
+
+test('failures', function () {
+    expect('d')->toBeIn(['a', 'b', 'c']);
+})->throws(ExpectationFailedException::class);