Etherlink/benchmark: fix issue introduced by new tx queue

What

This MR fixes an issue introduced by the switch to the node's tx queue in !20164 (merged). This restores the previous behavior.

Why

The call function would resolve immediately whereas the previous version resolved when the transaction was either confirmed, dropped, etc. This causes the Etherlink benchmarks to consider every operation included immediately, produce no block, and skip the whole benchmark.

How

Returns the confirmed promise instead of wrapping it in another promise.