components/sync/BUILD.gn: Clean up test_support_* targets

This moves some source files into the test_support target that they
belong to (e.g. from _driver to _engine or _model), and also adds a new
test_support_js target.

Bug: 947443
Change-Id: I09a4cbad9cedeb7c15ab871491f150f0149de417
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1570830
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653147}
2 files changed