[tests] Fix race in rpc_deprecated.py
[tests] Fix race in rpc_deprecated.py

Summary:
PR13130 backport https://github.com/bitcoin/bitcoin/pull/13130/files

Fixes a block generation race.

Test Plan:
    test_runner.py rpc_deprecated

Reviewers: Fabien, #bitcoin_abc, deadalnix

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4702

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