Fix a race in the net tests

Summary: There is a possible race that can occur when looping over the nodes while the opening connection thread is running. This can be fixed by joining the thread before running the check.

Test Plan: Run the tsan build.

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11833