Support cargo-xbuild

[vinaychandra]

Rust supports custom test frameworks (example). Enable test discovery and runner for the cross compiler used in such cases (cargo xbuild / cargo xrun here )

[calebcartwright]

Thanks for the request @vinaychandra! I've updated the title and scope of this issue to more narrowly focus specifically on xbuild/xtest because the processes required for test discover, execution, results interpretation, etc. are going to be specific to the corresponding test harnesses/frameworks.

We definitely want to be able to eventually support multiple test frameworks, but I want to be fully transparent that we're primarily focused on implementing the remaining features for libtest first, so it'll likely be some time before we get around to this one (though we'd happily review any PRs for this!).

As far as implementation for this (and really any custom test harness), #67 and #14 are both pre-requisites, and we'd also need to know:

• command are for test discovery and format of result
• command for test execution and format of result (with appropriate filters as needed)
• command for test suite/mod execution and format of result (with appropriate filters as needed)