[test] Fix wait condition in disconnect_p2ps

Summary:

MY_SUBVERSION is defined in messages.py as a byte string, but here we were comparing this value to the value returned by the RPC. Convert to ensure the types match.

Backport of core#20522.

This is the fix to the occasional failures in p2p_add_connections that can be seen on CI.

Test Plan:

for i in {1..100}
do
  ./test/functional/test_runner.py p2p_add_connections
done

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D10846