allow for simplified test data structures

Instead of a strict list of (URL, RESULTS)-tuples, extractor result
tests can now be a single (URL, RESULTS)-tuple, if it's just one test,
and "only matching" tests can now be a simple string.

gallery_dl/extractor/test.py

@@ -33,6 +33,7 @@ class TestExtractor(Extractor):
     """
     category = "test"
     pattern = [r"t(?:est)?:([^:]*)(?::([^:]*)(?::(\*|[\d,]*))?)?$"]
+    test = ("test:pixiv", "test:pixiv:user,favorite:0", "test:")
 
     def __init__(self, match):
         Extractor.__init__(self)
@@ -59,8 +60,7 @@ class TestExtractor(Extractor):
         tests = [
             test
             for extr in extractors
-            if hasattr(extr, "test")
-            for index, test in enumerate(extr.test)
+            for index, test in enumerate(extr._get_tests())
             if str(index) in self.indices
         ]