Use close=True in dump_snapshot to fix flaky tests

The dump_snapshot() method with close=False closes the node, runs hived
with --dump-snapshot --exit-before-sync, then restarts the node. Under
heavy CI load with parallel pytest workers, this restart can fail with
FailedToStartExecutableError due to a race condition where resources
(ports, files) aren't fully released.

Since test_loading_snapshot_from_other_node and
test_loading_snapshot_from_custom_path don't need the original node
after dumping, use close=True to skip the restart and avoid the race.