dataTestPrefixId usage is unintuitive

[mkarajohn]

Speaking with http://ictinus.herokuapp.com/?path=/docs/design-system-button--icon-button-sizes as a reference, the dataTestPrefixId prop does not indicate that a default value is used (which in truth it's button-)

Furthermore, even if dataTestPrefixId is passed a value, the final data-testid attribute will have the value <PREFIX>-button-<VALUE> which comes in contrast to the common expectation that the user-defined prefix would replace the default one (in this case <PREFIX> should replace button-)