Membership form test cleanup, date cleanup on form

[eileenmcnaughton]

Overview

This converts the test set up to use the api and also does some mild date clean up on the form.

Supports #18395

Before

Form uses variable variables, test uses brittle loadXMLDataSet

After

Form just uses it's noodle. Test uses api for setup

Technical Details

Comments

[civibot]

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[pradpnayak]

Tested via UI, join, start and end date are set correctly

[eileenmcnaughton]

thanks @pradpnayak !