fix(spanner): skip failing backup test (GoogleCloudPlatform#1184)

Author: larkee

spanner/test/spannerBackupTest.php

@@ -98,6 +98,10 @@ public function testCreateBackup()
      */
     public function testListBackupOperations()
     {
+        $this->markTestSkipped(
+            "See: https://github.com/GoogleCloudPlatform/php-docs-samples/issues/1186"
+        );
+
         $databaseId2 = self::$databaseId . '-2';
         $database2 = self::$instance->database($databaseId2);
         // DB may already exist if the test timed out and retried