Change single-test header format

Fixes @arv's review comment at https://codereview.chromium.org/771863002/diff/60001/test/mjsunit/harmony/array-includes.js.
Use three newlines between tests, not two

Addresses @arv's review comment at https://codereview.chromium.org/771863002/diff/60001/test/mjsunit/harmony/array-includes.js.