[tests/results] replace 'datetime' usage

test for 'dt:…' to catch NullDatetime instances,
which inherit from 'datetime.datetime' but are not exactly this class

test/results/mangapark.py

@@ -5,7 +5,6 @@
 # published by the Free Software Foundation.
 
 from gallery_dl.extractor import mangapark
-import datetime
 
 
 __tests__ = (
@@ -115,7 +114,7 @@ __tests__ = (
     "chapter"      : int,
     "chapter_id"   : r"re:\d+",
     "chapter_minor": str,
-    "date"         : datetime.datetime,
+    "date"         : "type:datetime",
     "lang"         : "en",
     "language"     : "English",
     "manga_id"     : 114972,