[bigint][test] Move bigint.js to bigint/basics.js

In preparation for adding more separate test files.

Bug: v8:6791
Change-Id: I92e2a3ab6fd35e2f902179a005ed9640851ca1b6
Reviewed-on: https://chromium-review.googlesource.com/716927Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48512}