Provide test coverage for field settings

Problem/Motivation

There is only test coverage for the field setting "Enable extension field". There should also be test coverage for "Allowed countries", "Allowed types" and "Set default value".

Steps to reproduce

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Proposed resolution

Add tests in PhoneNumberFieldFieldSettingsTest

Remaining tasks

Provide the tests
Review the tests

User interface changes

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API changes

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Data model changes

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