testrunner assumes that each test suite has a single extension for base tests.
".mjs" extension can be used for ECMAScript modules in addition to the standard
extension ".js" we use for the base tests.

This CL generalizes the {TestLoader} to accept multiple extensions for
a single test suite.

R=mathias@chromium.org
TBR=machenbach@chromium.org
CC=​gsathya@chromium.org

Bug: v8:9395
Change-Id: Ibc155f4963472fe9f989458cd839f3642ffbddea
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1675961Reviewed-by: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Auto-Submit: Tamer Tas <tmrts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62356}