Fix timezone unit test issue and add format test (#1043)

* Fix timestamp timezone in tests

* Set timezone for snippets

Core/snippet-bootstrap.php

@@ -3,3 +3,5 @@
 use Google\Cloud\Core\Testing\TestHelpers;
 
 TestHelpers::snippetBootstrap();
+
+date_default_timezone_set('UTC');

Core/tests/Unit/TimestampTest.php

@@ -208,4 +208,28 @@ public function timestampArrays()
             ]
         ];
     }
+
+    /**
+     * @dataProvider timezones
+     */
+    public function testTimezones($tz)
+    {
+        $time = new \DateTimeImmutable('now', new \DateTimeZone($tz));
+        $timestamp = new Timestamp($time);
+
+        $utc = $time->setTimeZone(new \DateTimeZone('UTC'));
+        $utcTs = new Timestamp($utc);
+
+        $this->assertEquals($utcTs->formatAsString(), $timestamp->formatAsString());
+    }
+
+    public function timezones()
+    {
+        return [
+            ['America/Detroit'],
+            ['Australia/Sydney'],
+            ['Pacific/Guam'],
+            ['Africa/Johannesburg'],
+        ];
+    }
 }

Core/unit-bootstrap.php

@@ -0,0 +1,3 @@
+<?php
+
+date_default_timezone_set('UTC');

phpunit.xml.dist

@@ -1,5 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit colors="true">
+<phpunit
+  bootstrap="./Core/unit-bootstrap.php"
+  colors="true">
   <testsuites>
     <testsuite>
       <directory>*/tests/Unit</directory>