[tests]: fix exponentiation in harmony array-concat tests
[tests]: fix exponentiation in harmony array-concat tests

BUG=
LOG=N
R=adamk@chromium.org

Review URL: https://codereview.chromium.org/921683006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#26710}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh