tests/docker: reduce scary warnings by cleaning up clean up

There was in the clean-up code caused by attempting to inspect images
which finished before we got there. Clean up the clean up code by:

  - only track the one instance at a time
  - use --filter for docker ps instead of doing it by hand
  - just call docker rm -f to be done with it
  - use uuid.uuid4() for a random uid

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

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