[backport#15963] [tests] Make random seed logged and settable

Summary:
[tests] Make random seed logged and settable (John Newbery)

Pull request description:

  This allows tests which use randomness to be reproducibly run on

failure.

https://github.com/bitcoin/bitcoin/pull/15963/files

Depends on D7081

Backport of Core PR15963

Test Plan:

ninja
cd test
../../test/functional/[test_name] --config=config.ini --randomseed=[random_seed]

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7083