Re-add dual_funding cfg-flag build & test

Author: dunxen

ci/ci-tests.sh

@@ -132,3 +132,5 @@ RUSTFLAGS="--cfg=taproot" cargo test --verbose --color always -p lightning
 RUSTFLAGS="--cfg=splicing" cargo test --verbose --color always -p lightning
 [ "$CI_MINIMIZE_DISK_USAGE" != "" ] && cargo clean
 RUSTFLAGS="--cfg=async_payments" cargo test --verbose --color always -p lightning
+[ "$CI_MINIMIZE_DISK_USAGE" != "" ] && cargo clean
+RUSTFLAGS="--cfg=dual_funding" cargo test --verbose --color always -p lightning