Fix abc_p2p_fullblocktest timeout with TSAN

Summary:
The very last part of the test is submitting a large block, which tends
to time out with the default 60s limit on the TSAN build.

Test Plan:
With TSAN:

ninja check-functional

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9120