testutil: Improve check_post_meta error reporting.

This version makes it easier to get diffs between the expected
and actual metadata.

tests/testutil.py

@@ -33,8 +33,10 @@ def check_post_meta(txn, *expected_meta, default=None):
         if not expected:
             assert not post.meta
         else:
-            assert all(post.meta.get(key, default) == value
-                       for key, value in expected.items())
+            actual = None if post.meta is None else {
+                key: post.meta.get(key, default) for key in expected
+            }
+            assert actual == expected
 
 def parse_date(s, fmt='%Y-%m-%d'):
     return datetime.datetime.strptime(s, fmt).date()