Don't rely on violating Shadow DOM in tests

[robdodson]

Tests that select elements inside of Web Components should query the Shadow Root.

This example: https://github.com/robdodson/angular-custom-elements/blob/master/src/directives/ce-interpolated/test/array-spec.js#L25

Should be rewritten to access the element's shadowRoot property and then querySelector inside of that for the wc-button