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:

silverstripe-framework/tests/php/Forms/FieldListTest.php

Lines 24 to 33 in cf78a9d

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