ContentTranslationSyncImageTest should use API to set up translation

Problem/Motivation

ContentTranslationSyncImageTest uses a form submission to set up field translation.

This is expensive, and not what the test is covering.

It should make API calls instead.

There is also this comment about checking the two admin pages agree:

    // Check that the content translation settings page reflects the changes
    // performed in the field edit page.

This is also not relevant to this test. It probably doesn't need moving elsewhere -- there are already tests for the admin/config/regional/content-language form correctly showing and saving config values, and the for the field edit form correctly showing and saving config values.

Steps to reproduce

Proposed resolution

Change to an API call.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet