[3.13] gh-137973: Add a non-parallel test plan to the iOS testbed project (GH-138018) (#138039)

Modifies the iOS testbed project to add a test plan. This simplifies the iOS
test runner, as we can now use the built-in log streaming to see test results.
It also allows for some other affordances, like providing a default LLDB config,
and using a standardized mechanism for specifying test arguments.
(cherry picked from commit 2ba2287b85eea3cc3a71d77c6bcf9eb5670ca05d)

Co-authored-by: Russell Keith-Magee <russell@keith-magee.com>