Merge #14985: test: Remove thread_local from test_bitcoin

Summary:
PR14985 backport https://github.com/bitcoin/bitcoin/pull/14985/files
fa61202cae test: Add comment to g_insecure_rand_ctx (MarcoFalke)
fa0d3c4407 test: Undo thread_local g_insecure_rand_ctx (MarcoFalke)

Pull request description:

`thread_local` seems to be highly controversial according to the discussion in #14953, so remove it again from the tests.

Also remove boost::thread_group in the test that uses it, since I am touching it anyway.

Backport note: corresponding parent in ABC was D3485; in Core it was
only five days before that was reverted with this PR.

Test Plan: make check

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3628