Bump the timeout for searching the debug log in p2p_ibd_stalling

Summary: The test sends a block and waits for the node to react by looking at the logs. The time spent processing the block might be larger than the default 2s on some slow CI machines, so bump the timeout to avoid random failures.

Test Plan:

./test/functional/test_runner.py p2p_ibd_stalling

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17596