Bug 1683797: Don't use "build targets" to build rusttests r=sheehan,firefox-build-system-reviewers,glandium

When "build targets" are used, the sccache server isn't
being used/integrated properly with make.
By moving rusttests to a separate build "application",
`client.mk` is invoked normally, and sccache is happy.

* Move rusttests to a new build "application".
* Move and denormalize rusttests mozconfigs. The new rusttests
  mozconfigs should expand to be mostly identical to their old
  versions, except with `MOZ_AUTOMATION_*=0` and
  `--enable-application=tools/rusttests`.
  They will differ a bit from their old variants because some
  of them originally didn't source from their
  respective `$app_mozconfigs/common`.
* Modify `baseconfig.mk` to change tiers if running rusttests.
* Adjust CI config to use new "rusttests" build application.

Differential Revision: https://phabricator.services.mozilla.com/D106294