Add a WrappingClock

The `WrappingClock` allows using an object with a `now()` method
returning a `DateTimeImmutable` object as a "real" clock.

tests/Clock/WrappingClockTest.php

@@ -0,0 +1,88 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Beste\Clock\Tests;
+
+use Beste\Clock\FrozenClock;
+use Beste\Clock\WrappingClock;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * @internal
+ * @coversDefaultClass \Beste\Clock\WrappingClock
+ */
+final class WrappingClockTest extends TestCase
+{
+    /**
+     * @test
+     *
+     * @covers ::__construct
+     * @covers ::create
+     * @covers ::now
+     */
+    public function itWrapsAClockInterface(): void
+    {
+        $clock = FrozenClock::fromUTC();
+
+        $wrappedClock = WrappingClock::wrapping($clock);
+
+        self::assertSame(
+            $clock->now()->format(DATE_ATOM),
+            $wrappedClock->now()->format(DATE_ATOM)
+        );
+    }
+
+    /**
+     * @test
+     *
+     * @covers ::__construct
+     * @covers ::create
+     * @covers ::now
+     */
+    public function itWrapsAnObjectWithANowMethod(): void
+    {
+        $now = FrozenClock::fromUTC()->now();
+
+        $clock = new class($now) {
+            private \DateTimeImmutable $now;
+
+            public function __construct(\DateTimeImmutable $now)
+            {
+                $this->now = $now;
+            }
+
+            public function now(): \DateTimeImmutable
+            {
+                return $this->now;
+            }
+        };
+
+        $wrappedClock = WrappingClock::wrapping($clock);
+
+        self::assertSame(
+            $now->format(DATE_ATOM),
+            $wrappedClock->now()->format(DATE_ATOM)
+        );
+    }
+
+    /**
+     * @test
+     *
+     * @covers ::create
+     */
+    public function itRejectsObjectsWithANowMethodReturningANonDateTimeImmutable(): void
+    {
+        $clock = new class() {
+            public function now(): string
+            {
+                return 'foo';
+            }
+        };
+
+        $this->expectException(\InvalidArgumentException::class);
+        $this->expectExceptionMessage('$clock->now() must return a DateTimeImmutable');
+
+        WrappingClock::wrapping($clock);
+    }
+}