Unexport HTTP proxy variables in autopkgtest

This works around lack of support for no_proxy, which breaks the tests on
Ubuntu autopkgtest. We don't actually need Internet access, so this is OK.

debian/tests/gnome-desktop-testing

@@ -3,4 +3,12 @@
 set -e
 exec 2>&1
 
+# Ubuntu provides internet access via a proxy, but libostree doesn't need
+# that. However, libostree also doesn't support no_proxy, so it will try
+# to use Ubuntu's proxy for localhost, and fail to reach itself.
+unset ftp_proxy
+unset http_proxy
+unset https_proxy
+unset no_proxy
+
 exec gnome-desktop-testing-runner libostree