Test for `gap-rule-overlap` property should be testing `rule-overlap`

[AtkinsSJ]

Specifically, https://github.com/web-platform-tests/wpt/blob/master/css/css-gaps/parsing/rule-overlap-valid.html, https://github.com/web-platform-tests/wpt/blob/master/css/css-gaps/parsing/rule-overlap-invalid.html, and https://github.com/web-platform-tests/wpt/blob/master/css/css-gaps/parsing/rule-overlap-computed.html actually check gap-rule-overlap:

wpt/css/css-gaps/parsing/rule-overlap-valid.html

Lines 15 to 16 in bac0545

	test_valid_value('gap-rule-overlap', 'row-over-column');
	test_valid_value('gap-rule-overlap', 'column-over-row');

It looks like this commit missed the fact that the gap- prefix had previously been removed: 7b085a8

What confuses me here is that Blink seems to still be treating gap-rule-overlap as the property name: https://wpt.fyi/results/css/css-gaps/parsing/rule-overlap-valid.html?label=experimental&label=master&aligned