Replace BAO calls with api calls in test class

[eileenmcnaughton]

Overview

Switches from direct BAO Membership::create to api in test class

Before

CRM_Member_BAO_Membership::create($params);

After

civicrm_api3('Membership', 'create', $params);

Technical Details

Working on getting rid of direct BAO calls as a step towards cleaning up the Membership create function so we can sort out v4

Test cover

[civibot]

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