test: Avoid rpc timeout in p2p_headers_sync_with_minchainwork

Summary:
Bump the timeout by a lot to avoid the rare JSONRPCException: 'generatetoaddress' RPC took longer than 60.000000 seconds. errors on CI.

This is a backport of core#26954

Test Plan:

mkdir build
cd build
cmake .. -GNinja
ninja
python ./test/functional/test_runner.py p2p_headers_sync_with_minchainwork

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15404