[tumblr] add tests for hidden and private blogs (#145)

Hidden / dashboard-only blogs are pretty straightforward and "only"
require a valid 'access-token' and 'access-token-secret' for the given
'api-key' and 'api-secret', so that signed OAuth1.0 requests are possible.

Private / password protected blogs on the other hand are a bit
cumbersome. In addition to a valid 'access-token' and
'access-token-secret', they also require the account belonging to those
tokens to be a member of the blog itself. Knowing the password and
entering it in the website isn't enough to access a blog through the
API. Following a private blog is also impossible, so that option can't
work either.

gallery_dl/extractor/tumblr.py

@@ -206,6 +206,18 @@ class TumblrUserExtractor(TumblrExtractor):
             "count": 3,
             "options": (("posts", "all"), ("external", True))
         }),
+        ("https://mikf123-hidden.tumblr.com/", {  # dashbord-only
+            "count": 2,
+            "keyword": {"tags": ["test", "hidden"]},
+        }),
+        ("https://mikf123-private.tumblr.com/", {  # password protected
+            "count": 2,
+            "keyword": {"tags": ["test", "private"]},
+        }),
+        ("https://mikf123-private-hidden.tumblr.com/", {  # both
+            "count": 2,
+            "keyword": {"tags": ["test", "private", "hidden"]},
+        }),
         ("https://demo.tumblr.com/page/2", None),
         ("https://demo.tumblr.com/archive", None),
         ("tumblr:http://www.b-authentique.com/", None),