Improve test coverage in FieldListTest

[sabina-talipova]

Description

• test for FieldList->setValues(). Need to check that the values that were set are the correct ones given back.
• test for FieldList->transform() and FieldList->makeReadonly(). Need to ensure that it correctly transforms the FieldList object.
• test for FieldList->HiddenFields(). Need to check the fields returned are the correct HiddenField objects for a given FieldList instance.
• test for FieldList->dataFields().
• test for FieldList->findOrMakeTab() the same as above with insertBefore() and insertAfter()

See: https://github.com/silverstripe/silverstripe-framework/blob/cf78a9d785b9959a92dfb52c2a456bea80a4c567/tests/php/Forms/FieldListTest.php#L24-L33