Fix failing PHP test in WP nightly

[nfmohit]

Bug Description

Due to a recent change in WordPress, one of our PHP tests has started failing in the nightly build.

• Example failure
• Failing test case

It looks like the test case tries to mutate the roles property in the WP_User object, which has now been changed to a protected property.

We should use the provided class methods to set roles.

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Acceptance criteria

• The failing test case highlighted in the issue description should be updated so that it passes in all PHP versions and WordPress builds.

Implementation Brief

Test Coverage

QA Brief

Changelog entry