Filter platforms in 'run_genode_until'

[Martin Stein]

Currently i often run tests that are executable in qemu
only on certain platforms but shall be build through the
scripts for others as well to do testing on hardware.
Would it be nice to let 'run_genode_until' filter out
platforms that are not supported autonomously, instead of
redundantly guard it via "if have spec" lists in any test?

Greets,
Martin