Fix an occasional failure in abc_p2p_fullblocktest

Summary: The last action of the test is to submit a lare block by RPC. The node will attempt to relay that block to its peers which can cause a shutdown timeout on slow machines. Disconnecting the peers before calling the RPC should fix it.

Test Plan:

./test/functional/test_runner.py abc_p2p_fullblocktest

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12027