testsuite: Unset https_proxy and http_proxy

This breaks the autopkgtest on Ubuntu, as they use a proxy, and
that responds with 403.

testsuite

@@ -28,6 +28,9 @@ set -e
 # for handling jobspecs:
 set -m
 
+# Unset proxy to make sure curl behaves correctly
+unset https_proxy http_proxy
+
 hddemux=$(which hddemux) || hddemux=./hddemux
 
 [ -x "$hddemux" ]