Add support for tests declaring themselves to have been skipped

This is a useful way to represent tests that cannot always run, for reasons that are too complex or unusual to put in Restrictions. For example, the flatpak and ostree tests require that /var/tmp supports extended attributes.

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This would also have been useful for https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=901030 which involves tests that require that binfmt_misc works.

Based on and includes !19 (merged): they touch the same code, so developing them orthogonally would be awkward.