Fix multiwallet to use self.assert_start_raises_init_error

Summary:
Multiwallet was using the incorrect assert_start_raises_init_error
due to a diff ordering issue when landing.

Fixes T195

Test Plan: make && test/functional/test_runner.py multitest

Reviewers: deadalnix, matiu, jasonbcox, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Maniphest Tasks: T195

Differential Revision: https://reviews.bitcoinabc.org/D1025