Fix has-slotted-functional-changing-00*.tentative.html subtest names (#48615)
Using the testharness window's name as a subtest name is problematic
because WPT doesn't prescribe any particular value. For some browsers,
wpt run sets name to an unstable UUID 0 (wpt.fyi results 1).
Simply omit the name arg to default to document.title instead 2.