Move call to SetRPCWarmupFinished() so that it applies to the entire test framework

Summary:
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.

Test Plan:

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