[NTP Tiles] Add tests for context menu usage

Follow up to https://codereview.chromium.org/2758753003/

BUG=701253

Review-Url: https://codereview.chromium.org/2754203004
Cr-Commit-Position: refs/heads/master@{#458127}
12 files changed