test: Fix intermittent wallet_multiwallet issue with got_loading_error

Summary:
Failing the test after 10 iterations without a loading error is problematic because it may take 11 iterations to get a loading error.

Fix that by running until a loading error occurs, which should happen in almost all runs within the first 10 iterations.

This is a backport of core#20569

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10684