[Test] [NFC] upgrade remaining export tests

[eileenmcnaughton]

Overview

Minor cleanup on Export test & remove a few lines from core fn which supported hackier test method

Before

Older helper used, more hacky

After

Newer helper used, hack removed

Technical Details

Fully switch to checking csv output with new helper - this allows us to remove the hack from the core fn

Comments

[civibot]

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[seamuslee001]

This is fine and talked it through with Eileen, plenty of tests already in this area merging