test: Use Popen.wait instead of RPC in assert_start_raises_init_error

Summary:

Using RPC (wait_for_rpc_connection) has several issue:

 It polls in a loop, which might be slow
It tries to read the RPC cookie file, which might not be present, thus leading to intermittent issues

Fix both by using Popen.wait

This is a backport of core#20613

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10897