[REF][PHP8.2] Avoid dynamic properties in module tests

[braders]

Overview

Avoid dynamic properties in module tests

Before

basedir and system were used as dynamic properties, not declared.

After

The properties are declared, we move closer to PHP8.2 support.

See https://lab.civicrm.org/dev/core/-/issues/3833

[civibot]

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[eileenmcnaughton]

Yep - as with other abbreviatsions basedir can be challenging for non-English speakers to interpret (probably with camel case they would be able to guess dir = Directory) - but that is unchanged