components/sync/BUILD.gn: Introduce test_support group target

This CL introduces a new "test_support" target in
components/sync/BUILD.gn, so that other code can just depend on this
instead of all the individual "test_support_x" targets.
This lets us move the individual targets into their own build files
(so far, this was done for test_support_base, _js, and _driver),
without having to update the universe each time.

TBR=blundell
for mechanical changes in components/*/BUILD.gn

Bug: 947443
Change-Id: Ibe87bd82caab07b562c052a230f32539f8874792
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1581634
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653962}
24 files changed