[REF] extract member form batch update & add test

[eileenmcnaughton]

Overview

Unit test from exploration into #16116

Before

No test

After

test

Technical Details

I noticed & fixed the setting of 'id' before being passed through to add. I haven't investigated but this might be a recent regression. Otherwise straight extraction

Comments

[civibot]

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[colemanw]

Code looks good & this adds a test.