fix(get-role): handle presentation role inheritance for vnodes with no parent

[dbowling]

Closes issue: #3788

[WilcoFiers]

LGTM, although it'd be good to add a couple more tests to prove this out.

[WilcoFiers]

Can you add a couple more tests:

With empty content:

1. it passes when the table has role=none
2. it passes when the th has role=cell

[dbowling]

it passes when the table has role=none

Steve said that the following test needs to be a new ticket as it is not a related issue. He was going to chat with you today about it I think.

This test currently fails:

it('should pass with a table of role none', function () {
	var table = new axe.SerialVirtualNode({
		nodeName: 'table',
		attributes: {
			role: 'none'
		}
	});

	var tr = new axe.SerialVirtualNode({
		nodeName: 'tr',
	});

	var th = new axe.SerialVirtualNode({
		nodeName: 'th',
	});

	tr.children = [th];
	tr.parent = table;
	th.parent = tr;
	th.children = [];
	table.children = [tr];

	var results = axe.runVirtualRule('empty-table-header', table);

	assert.lengthOf(results.passes, 1);
	assert.lengthOf(results.violations, 0);
	assert.lengthOf(results.incomplete, 0);
});

it passes when the th has role=cell

Test has been added and passes.

[WilcoFiers]

Please open that follow-up issue on table[role=none].

[dbowling]

Issue added, #3817