Test sidebars API based on capabilities, not roles

[adamziel]

At the moment, the unit tests for the sidebars endpoint are checking for "admin", "editor", "author", "subscriber" roles:

gutenberg/phpunit/class-rest-sidebars-controller-test.php

Lines 163 to 168 in cefab03

	public function test_get_items_wrong_permission_author() {
	wp_set_current_user( self::$author_id );
	$request = new WP_REST_Request( 'GET', '/__experimental/sidebars' );
	$response = rest_get_server()->dispatch( $request );
	$this->assertErrorResponse( 'widgets_cannot_access', $response, 403 );
	}

It would be way better to test based on the capabilities (edit_theme_options maybe?).