Fixed up the webp leak check

Tests/check_webp_leaks.py

@@ -1,38 +1,25 @@
-from __future__ import division
-from helper import unittest, PillowTestCase
-import sys
-from PIL import Image
+from helper import unittest, PillowLeakTestCase
+from PIL import Image, features
 from io import BytesIO
 
-# Limits for testing the leak
-mem_limit = 16  # max increase in MB
-iterations = 5000
 test_file = "Tests/images/hopper.webp"
 
+@unittest.skipUnless(features.check('webp'), "WebP is not installed")
+class TestWebPLeaks(PillowLeakTestCase):
 
-@unittest.skipIf(sys.platform.startswith('win32'), "requires Unix or MacOS")
-class TestWebPLeaks(PillowTestCase):
-
-    def setUp(self):
-        try:
-            from PIL import _webp
-        except ImportError:
-            self.skipTest('WebP support not installed')
-
-    def _get_mem_usage(self):
-        from resource import getpagesize, getrusage, RUSAGE_SELF
-        mem = getrusage(RUSAGE_SELF).ru_maxrss
-        return mem * getpagesize() / 1024 / 1024
+    mem_limit = 3 * 1024 # kb
+    iterations = 100
 
     def test_leak_load(self):
         with open(test_file, 'rb') as f:
             im_data = f.read()
-        start_mem = self._get_mem_usage()
-        for _ in range(iterations):
+
+        def core():
             with Image.open(BytesIO(im_data)) as im:
                 im.load()
-            mem = (self._get_mem_usage() - start_mem)
-            self.assertLess(mem, mem_limit, msg='memory usage limit exceeded')
+
+        self._test_leak(core)
+
 
 if __name__ == '__main__':
     unittest.main()