The cross-channel import tests from #54002 hardcoded
`ReleaseChannel::Preview` and `ReleaseChannel::Nightly` as source
channels. When those tests run on a release branch (where the current
channel is Preview or Nightly), the import skips the current channel,
causing test failures.

This adds a `foreign_channels` helper that dynamically picks two
channels that are neither the current one nor Dev, making the tests work
on main, preview, nightly, or any other branch.

Discovered while cherry-picking #54002 to v0.233.x (#54021).

Release Notes:

- N/A