[test] Make test262 testing variant explicit

It was confusing to silently run test262 only in the default variant with
a switch on the infra side. We'll remove that switch in a follow up and
explicitly configure the testing variant for test262 in builders.pyl.

Bug: v8:9791
Change-Id: I0da351ff981c833704f51a415225aff24df1664a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1829269Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64055}