Now that the fuzz tests can use the BasicTestingSetup [1], do the same for bench.
Partial backport of core PR15788:
https://github.com/bitcoin/bitcoin/pull/15788/commits/fa8685d49ed8e52b1220a89f669dac63bef172bc
This is stripped down to using the test fixture for the benchmark, as we
have no use for the utils for now.
Depends on D5484.