bench: Make CoinSelection output groups pass eligibility filter

Summary:
Set the depth of the output groups used in the CoinSelection benchmark
to be 6 in order to pass the eligibility filter for the benchmark.

Backport Core PR13822

Test Plan:

src/bench/bench_bitcoin --filter=CoinSelection

Ensue this is not segfaulting anymore.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3428