gnu: knot: Fix test failure on aarch64-linux.

* gnu/packages/dns.scm (knot)[source]: Apply patch from upstream that
helps ensure the "test_net_shortwrite" test suite can complete
regardless of platform or configuration.
* gnu/packages/patches/knot-test_net_shortwrite-ensure-connection.patch:
New file.
* gnu/local.mk (dist_patch_DATA): Add it.

Signed-off-by: Tobias Geerinckx-Rice <me@tobias.gr>