PointerEvent: move .fromElement/.toElement checks to an optional WPT (#57300)

The `.fromElement` and `toElement` attributes are mentioned in the spec
as optional and therefore moved to a separate WPT named ".optional".

This is our second attempt to fix the test for those attributes, the
first one kept the optional part in the same html and was reverted:
#57202

This CL also simplifies the original WPT, without losing coverage, to
replace historical crafts with a cleaner promise-based logic.

Change-Id: Ifd76f128700b5880780022db118e2fba7eab2dd6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7489239
Auto-Submit: Mustaq Ahmed <mustaq@chromium.org>
Commit-Queue: Robert Flack <flackr@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1573305}

Co-authored-by: Mustaq Ahmed <mustaq@google.com>

Author: chromium-wpt-export-bot