fix: populate ldns submodule and add autotools to LDNS build stage

- Re-cloned zonemaster-ldns with --recurse-submodules so the bundled
  ldns C library source (including Changelog and configure.ac) is present
- Added autoconf, automake, libtool to Dockerfile.backend ldns-build stage
  so libtoolize + autoreconf can generate ldns/configure during make

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

zonemaster-ldns/ldns/test/testbed/do-tests.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+TPKG=/home/jeltejan/repos/tpkg/tpkg
+
+NEED_SPLINT='00-lint.tpkg'
+NEED_DOXYGEN='01-doc.tpkg'
+
+cd testdata;
+for test in `ls *.tpkg`; do
+	SKIP=0
+	if echo $NEED_SPLINT | grep $test >/dev/null; then
+		if test ! -x "`command -v splint`"; then
+			SKIP=1;
+		fi
+	fi
+	if echo $NEED_DOXYGEN | grep $test >/dev/null; then
+		if test ! -x "`command -v doxygen`"; then
+			SKIP=1;
+		fi
+	fi
+	if test $SKIP -eq 0; then
+		echo $test
+		$TPKG -a ../.. exe $test
+	else
+		echo "skip $test"
+	fi
+done