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This introduces assertFocusMoves() that helps us to write
testharness.js-tests with the compact and declarative "step focus and
check result"-syntax of the older snav*-tests.

I've converted snav-div-scrollable-but-without-focusable-content.html
and snav-radio.html so these do no longer need -expected.txt-files,
asserts are now made through testharness.js instead.
Other snav*-tests could be converted by following their example.

Bug: 803086
Change-Id: I5310f16492b233f894eda7e3e6b9691926670b06
Reviewed-on: https://chromium-review.googlesource.com/998193Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Hugo Holgersson <hugoh@vewd.com>
Cr-Commit-Position: refs/heads/master@{#581570}