Fix debug build test failures

Summary:
Running the tests with debug enabled would fail due to the
AssertLockNotHeld(cs_main) introduced in D2608. Some tests are still
locking cs_main when creating a new block, which now throws an
assertion error.
This diff fixes these tests.

Test Plan:

../configure --enable-debug && make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2634