test: fix flaky test-net-write-slow

Checklist

• make -j4 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

test net

Description of change

Increase socket timeout so that there is enough time to reliably run the test on FreeBSD.

Fixes: https://github.com/nodejs/node/issues/7516