Just copies the StringConcat tests and refactors them to exercise
template literals rather than simple string addition.

BUG=v8:7415
R=rmcilroy@chromium.org

Change-Id: I79cf24ee33e64b1d57221eb0291d9958634130ec
Reviewed-on: https://chromium-review.googlesource.com/951968Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#51788}