[REF][PHP8.2] Tidy up properties on api_v3_GroupContactTest

[braders]

Overview

Tidy up properties on api_v3_GroupContactTest, with the primary purpose of removing dynamic properties.

Before

_group was a dynamic property, deprecated in PHP 8.2.

After

No longer any dyamic properties in use.

Properties have been renamed for better readability, and consistency with the rest of the CiviCRM codebase.

[civibot]

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