Missing timer tests as pointed out by @clue at reactphp#112 (comment)

tests/Timer/ExtUvTimerTest.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace React\Tests\EventLoop\Timer;
+
+use React\EventLoop\ExtUvLoop;
+
+class ExtUvTimerTest extends AbstractTimerTest
+{
+    public function createLoop()
+    {
+        if (!function_exists('uv_loop_new')) {
+            $this->markTestSkipped('uv tests skipped because ext-uv is not installed.');
+        }
+
+        return new ExtUvLoop();
+    }
+}