compiletest: investigate if it's possible to do run-time capability detection against *target*

[jieyouxu]

Some tests are conditioned on capabilities, which are most accurately detected at run-time against the target under cross-compilation scenarios (compared to having to maintain allowlists or denylists of targets). However, implementing this is non-trivial:

• You have to build and run the capability run-time test as an executable on the target, not the host. This may involve having to work with custom test runners (e.g. wasmtime) or remote-test-server.
• The target may not support std!