Make sure netrc works for all extractors with login support

Fixes #5112

test/test_netrc.py

@@ -0,0 +1,26 @@
+# coding: utf-8
+from __future__ import unicode_literals
+
+import os
+import sys
+import unittest
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+
+from youtube_dl.extractor import (
+    gen_extractors,
+)
+
+
+class TestNetRc(unittest.TestCase):
+    def test_netrc_present(self):
+        for ie in gen_extractors():
+            if not hasattr(ie, '_login'):
+                continue
+            self.assertTrue(
+                hasattr(ie, '_NETRC_MACHINE'),
+                'Extractor %s supports login, but is missing a _NETRC_MACHINE property' % ie.IE_NAME)
+
+
+if __name__ == '__main__':
+    unittest.main()