Leave IBD in rpc_listtransactions

Summary:
The test won't run alone because IBD will prevent the inital
sync_all() to sync the mempools.

Test Plan:

./test/functional/test_runner.py rpc_listtransactions

Will fail before the diff, but not after.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2628