Allow moving tests into IE files

Allow adding download tests right in the IE file.
This will cut down on merge conflicts and make it more likely that new IE authors will add tests right away.

test/test_download.py

@@ -58,6 +58,13 @@ def _file_md5(fn):
 
 with io.open(DEF_FILE, encoding='utf-8') as deff:
     defs = json.load(deff)
+for ie in youtube_dl.extractor.gen_extractors():
+    t = getattr(ie, '_TEST', None)
+    if t:
+        t['name'] = type(ie).__name__[:-len('IE')]
+        defs.append(t)
+
+
 with io.open(PARAMETERS_FILE, encoding='utf-8') as pf:
     parameters = json.load(pf)