test(cluster): configure wal_threads_root for t/353 reborn HW remaster

t/353 L5 crashes node0 and has the reborn node take a FRESH HW space lease,
which requires the survivor to complete HW remaster of the dead node's
shards.  HW remaster reads the dead node's per-thread WAL, so on this
shared_data 2-node pair it is only recoverable when cluster.wal_threads_dir
is configured (cluster_hw_remaster_recoverable).  The pair was created with
shared_data but WITHOUT wal_threads_root, so recoverable() returned false,
the remaster was marked BLOCKED_STRUCTURAL, and the reborn node's fresh-lease
DDL/insert/count fail-closed (L5 subtests 28-31, nightly crossnode-b).

Opt the pair into wal_threads_root, matching every other HW-remaster-
dependent kill test (t/247/248/274/293).  This completes the per-thread WAL
harness config that the RACvsRAC/ClusterQuad harness already carries but this
ClusterPair test was missing.

Spec: spec-4.6a-grd-recovery-liveness.md

Author: SqlRush

src/test/cluster_tap/t/353_cluster_6_12d_space_lease.pl

@@ -117,6 +117,15 @@ sub poll_until
 	'd612_lease',
 	quorum_voting_disks => 3,
 	shared_data         => 1,
+	# The L5 reborn leg takes a FRESH HW space lease, which needs the survivor
+	# to complete HW remaster of the dead node's shards.  HW remaster reads the
+	# dead node's per-thread WAL, so on a shared_data multi-node pair it is only
+	# recoverable when cluster.wal_threads_dir is set
+	# (cluster_hw_remaster_recoverable); without it the remaster is marked
+	# BLOCKED_STRUCTURAL and the reborn node's fresh-lease access stays
+	# fail-closed.  Every HW-remaster-dependent kill test opts into this
+	# (t/247/248/274/293).  Spec: spec-4.6a-grd-recovery-liveness.md (D4).
+	wal_threads_root    => 1,
 	extra_conf          => [
 		'autovacuum = off',
 		'jit = off',