Merge #14822: bench: Destroy wallet txs instead of leaking their memory

Summary:
PR14822 backport

fa5cef0f78 bench: Destroy wallet txs instead of leaking their memory (MarcoFalke)

Pull request description:

This should destroy the wallet txs when the benchmark ends to avoid having to hold them when the following benchmarks run.

Depends on D5251 (only for having updated valgrind suppressions, besides this it is landable anytime)

Test Plan:
ninja bench-bitcoin

Run the following and observe there are no more leaks--

ninja src/bench/bitcoin-bench
valgrind --suppressions=../contrib/valgrind.supp src/bench/bitcoin-bench -evals=1 -scaling=0.0

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D5252