[iOS] Mark test bundle_data targets as testonly.

Mark bundle_data targets used only for tests targets as testonly
to prevent linking them in production applications.

BUG=297668

Review URL: https://codereview.chromium.org/1807213002

Cr-Commit-Position: refs/heads/master@{#382199}
4 files changed