[qa] zapwallettxes: Wait up to 3s for mempool reload

Summary: backport core's PR11308 ( fadd0c1 )

Test Plan:

./test/functional/test_runner.py zapwallettxes

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1160