Bug 1687719 [wpt PR 27256] - Update SlotAssignmentMode to name fr…

…om `auto` to match the latest proposal, a=testonly

Automatic update from web-platform-tests
Update `SlotAssignmentMode` to `name` from `auto` to match the latest proposal (#27256)

[By the latest proposal](whatwg/dom#860),
`SlotAssignmentMode` value is changed to `name` from `auto`.

This change follows it.
--

wpt-commits: 875f4c73ec1122cd0ea05580f6b56fda0ef71cc7
wpt-pr: 27256

UltraBlame original commit: 84ac1dad84575a71ebc6ad87ce7a6f71d6a1f864

testing/web-platform/tests/shadow-dom/slots-imperative-slot-api.tentative.html

@@ -206,7 +206,7 @@
 slotAssignment
 :
 '
-auto
+name
 '
 }
 )
@@ -303,7 +303,7 @@
 assignment
 =
 "
-auto
+name
 "
 >
 <