[text-autospace][text-spacing-trim] Update `get-char-advances.js`

This patch updates `getCharAdvances` function used by tests
for `text-autospace` and `text-spacing-trim`, so that it uses
the differences between right edges of bounding boxes instead
of the widths of bounding boxes.

There are two ways to adjust spacing; one is to adjust width,
and another is to move the glyph. When the latter happens,
the width of the bounding box doesn't change, and the logic
before this change could not detect.

The new logic is needed for future patches, but this patch is
split to ensure the change doesn't affect existing tests.

Bug: 1463890, 1463891
Change-Id: I2619510b59ff8732f8416d44d676d4808127610d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4918498
Auto-Submit: Koji Ishii <kojii@chromium.org>
Reviewed-by: Lingqi Chi <lingqi@chromium.org>
Commit-Queue: Lingqi Chi <lingqi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1207441}