Convert editing/selection/modify_move/move-by-word-visually-single-space-one-element.html with selection_test

This patch converts the web test with selection_test() so that the test
cases are more explicit and the expectations are easier to maintain.

This is a preparation for switching to logical left/right caret movement:
crrev.com/c/1593632

Bug: 958831
Change-Id: I1a48469f71c269c0b752e2540a79d3eeb87f1dda
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1594619
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Auto-Submit: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657287}