Mark slow tests

Each of these tests took more than ten seconds on a recent AppVeyor CI
run; that appears to qualify them to be marked as slow tests.

Closes GH-7077.

ext/mysqli/tests/bug79375.phpt

@@ -2,6 +2,7 @@
 Bug #79375: mysqli_store_result does not report error from lock wait timeout
 --SKIPIF--
 <?php
+if (getenv("SKIP_SLOW_TESTS")) die("skip slow test");
 require_once('skipif.inc');
 require_once('skipifconnectfailure.inc');
 if (!defined('MYSQLI_STORE_RESULT_COPY_DATA')) die('skip requires mysqlnd');

ext/mysqli/tests/mysqli_fetch_array_large.phpt

@@ -2,6 +2,7 @@
 mysqli_fetch_array() - large packages (to test compression)
 --SKIPIF--
 <?php
+if (getenv("SKIP_SLOW_TESTS")) die("skip slow test");
 require_once('skipif.inc');
 require_once('skipifconnectfailure.inc');
 ?>

ext/pdo_mysql/tests/bug79375.phpt

@@ -2,6 +2,7 @@
 Bug #79375: mysqli_store_result does not report error from lock wait timeout
 --SKIPIF--
 <?php
+if (getenv("SKIP_SLOW_TESTS")) die("skip slow test");
 if (!extension_loaded('pdo') || !extension_loaded('pdo_mysql')) die('skip not loaded');
 require_once(__DIR__ . DIRECTORY_SEPARATOR . 'skipif.inc');
 require_once(__DIR__ . DIRECTORY_SEPARATOR . 'mysql_pdo_test.inc');

ext/standard/tests/file/005_variation-win32.phpt

@@ -5,6 +5,7 @@ Test fileatime(), filemtime(), filectime() & touch() functions : usage variation
 if (substr(PHP_OS, 0, 3) != 'WIN') {
     die('skip Windows-only test');
 }
+if (getenv("SKIP_SLOW_TESTS")) die("skip slow test");
 ?>
 --FILE--
 <?php