Make unit tests use the insecure_rand_ctx exclusively

Summary:
This is a partial backport of Core PR14624 : https://github.com/bitcoin/bitcoin/pull/14624/commits/fd3e7973ffaaa15ed32e5aeadcb02956849b8fc7

Depends on D3437 and D3441

Test Plan:

make check

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3450