Fix the extensions of two tests with youtube videos

The best quality is now a mp4 video.

youtube_dl/extractor/metacafe.py

@@ -23,7 +23,7 @@ class MetacafeIE(InfoExtractor):
     _TESTS = [{
         u"add_ie": ["Youtube"],
         u"url":  u"http://metacafe.com/watch/yt-_aUehQsCQtM/the_electric_company_short_i_pbs_kids_go/",
-        u"file":  u"_aUehQsCQtM.flv",
+        u"file":  u"_aUehQsCQtM.mp4",
         u"info_dict": {
             u"upload_date": u"20090102",
             u"title": u"The Electric Company | \"Short I\" | PBS KIDS GO!",

youtube_dl/extractor/youtube.py

@@ -348,7 +348,7 @@ class YoutubeIE(YoutubeBaseInfoExtractor, SubtitlesInfoExtractor):
         },
         {
             u"url":  u"http://www.youtube.com/watch?v=1ltcDfZMA3U",
-            u"file":  u"1ltcDfZMA3U.flv",
+            u"file":  u"1ltcDfZMA3U.mp4",
             u"note": u"Test VEVO video (#897)",
             u"info_dict": {
                 u"upload_date": u"20070518",