bpo-33901: Better test_dbm_gnu.test_reorganize() fix (GH-7795)

Fix test_dbm_gnu.test_reorganize() on macOS with gdbm 1.15: add a
larger value to make sure that the file size changes.

Lib/test/test_dbm_gnu.py

@@ -72,9 +72,13 @@ class TestGdbm(unittest.TestCase):
         self.g = gdbm.open(filename, 'c')
         size0 = os.path.getsize(filename)
 
-        self.g['x'] = 'x' * 10000
+        # bpo-33901: on macOS with gdbm 1.15, an empty database uses 16 MiB
+        # and adding an entry of 10,000 B has no effect on the file size.
+        # Add size0 bytes to make sure that the file size changes.
+        value_size = max(size0, 10000)
+        self.g['x'] = 'x' * value_size
         size1 = os.path.getsize(filename)
-        self.assertGreaterEqual(size1, size0)
+        self.assertGreater(size1, size0)
 
         del self.g['x']
         # 'size' is supposed to be the same even after deleting an entry.
@@ -82,7 +86,7 @@ class TestGdbm(unittest.TestCase):
 
         self.g.reorganize()
         size2 = os.path.getsize(filename)
-        self.assertLessEqual(size2, size1)
+        self.assertLess(size2, size1)
         self.assertGreaterEqual(size2, size0)
 
     def test_context_manager(self):

Misc/NEWS.d/next/Tests/2018-06-19-14-04-21.bpo-33901.OFW1Sr.rst

-Fix test_dbm_gnu for gdbm 1.15. Using gdbm 1.15, creating a database creates
-a file of 16 MiB. Adding a small entry and then modifying the small entry
-doesn't change the file size. Modify test_dbm_gnu to be less strict: allow
-that the file size doesn't change.
+Fix test_dbm_gnu on macOS with gdbm 1.15: add a larger value to make sure that
+the file size changes.