[tests] Prepare CSS.escape() tests for upcoming JSON.stringify() change

The well-formed JSON.stringify() proposal changes JSON.stringify() to
produce JavaScript/JSON-compatible escape sequences for any lone
surrogates in its input.

Because the test runner's `shouldBeEqualToString` uses `JSON.stringify`
before comparing strings, these tests would otherwise break when
shipping the `JSON.stringify()` change.

Bug: v8:7782
Change-Id: If6ace9c023be8997ed10c8f1dbc2dcaed18c4687
Reviewed-on: https://chromium-review.googlesource.com/c/1281965
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599922}
2 files changed