Bump timeout when checking contiguous addresses in net_tests

Summary: The 10s timeout might be a bit low for the last case as it's only ~170ms per address, which might not be enough on very slow machines like when running TSAN.

Test Plan:

ninja check-bitcoin-net_tests

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16245