Move call to SetRPCWarmupFinished() so that it applies to the entire test framework
Encountered while writing D1894.
SetRPCWarmupFinished() needs to be called by the test framework since it is normally called via bitcoind's init cycle. Calling it in test_bitcoin is more appropriate since tests should expect the RPC to be in post-warmup by default, as the test framework already registers RPC commands.
make check ninja check
Both on master and on D1894.
Reviewers: deadalnix, Fabien, #bitcoin_abc
Reviewed By: deadalnix, #bitcoin_abc
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2507