(mosaico#247) (dev/core#588) Apply prefix to subject of test mailings

[totten]

This is was previously fixed in https://github.com/civicrm/civicrm-core/pull/12758, but that fix had a couple undesired side-effect:

• For non-Mosaico mailings, the prefix would be applied twice (https://lab.civicrm.org/dev/core/issues/588); once in the JS layer and once in the BAO delivery engine.
• The saved value of the subject would temporarily store the prefix as part of the subject. (In normal usage, it would be fixed later by an autosave; but there could be edge-cases/races.)

This is an alternative fix which makes flexmailer more precisely imitate the behavior in the BAO delivery engine.

Note: If one doesn't like this behavior, then you can disable or replace the service, e.g.

Civi::service('civi_flexmailer_test_prefix')->setActive(FALSE);

[totten]

I've r-run this locally (along with a revert of 12758) and confirmed the expected subject lines for:

• Normal mailing => Send test => specific email (with prefix)
• Normal mailing => Send test => group (with prefix)
• Normal mailing => Actual delivery (*without prefix)
• Mosaico mailing => Send test => specific email (with prefix)
• Mosaico mailing => Send test => group (with prefix)
• Mosaico mailing => Actual delivery (*without prefix)

[seamuslee001]

@totten i have merged the other PR so i think this is good now

[totten]

/test

[totten]

Thanks, @seamuslee001.

Tests have passed here. Merging.