Bug 1263665 - Set media.libavcodec.allow-obsolete=true for testing - r=jya

Separate patch as we may want to revert it in the future, when test systems
have been upgraded with a more recent libavcodec.

MozReview-Commit-ID: EUrdsb8UGnV

--HG--
extra : rebase_source : 5bec3f1834945a25da411ab5d2e8885b903f5998

testing/profiles/prefs_general.js

@@ -357,3 +357,7 @@ user_pref("browser.usedOnWindows10.introURL", "");
 // For Firefox 52 only, ESR will support non-Flash plugins while release will
 // not, so we keep testing the non-Flash pathways
 user_pref("plugin.load_flash_only", false);
+
+// Don't block old libavcodec libraries when testing, because our test systems
+// cannot easily be upgraded.
+user_pref("media.libavcodec.allow-obsolete", true);