Re-enable libostree/test-concurrency.py.test, but mark it as flaky

See #901170.

debian/tests/control

@@ -1,3 +1,7 @@
+Tests: flaky
+Restrictions: flaky
+Depends: gnome-desktop-testing, ostree-tests
+
 Tests: gnome-desktop-testing
 Depends: gnome-desktop-testing, ostree-tests
 

debian/tests/flaky

@@ -0,0 +1,14 @@
+#!/bin/sh
+
+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/test-concurrency.py.test