The guest mutates its boot volume (fat tests create files, the logger
writes /var/log) and QEMU is hard-killed after a match, so booting the
build artifact in place let one run's leftovers fail the next — a stale
TESTDIR tripping the mkdir-duplicate refusal was the lone 87/88 failure
in an otherwise green suite — and dirtied the build cache's own output.
Each case now boots a fresh copy in the work directory.
Daniel Samson
4091ea6912