faead93c6c test: Make g_insecure_rand_ctx thread_local (MarcoFalke)
Pull request description:
Some tests might spin up several threads and `FastRandomContext` is not thread safe.
Fix that by giving each thread their own randomness context (as opposed to e.g. making `FastRandomContext` thread safe or add locks elsewhere).
Also, add the `g_` prefix to it (according to developer notes), since I am touching it anyway.
https://github.com/bitcoin/bitcoin/pull/14953/commits/faead93c6c7e1d27bd2a1b2a9b496fda838c2996
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Backport of Core PR14953