Merge remote-tracking branch 'remotes/stsquad/tags/pull-testing-060121-4' into staging

Testing updates (back to green)

  - include ccache in Debian 10 docker image
  - iotests: drop 312 from auto group
  - bound reading of s390x framebuffer file
  - cirrus: drop non-x86 tests so we complete

# gpg: Signature made Wed 06 Jan 2021 17:31:14 GMT
# gpg:                using RSA key 6685AE99E75167BCAFC8DF35FBD0DB095A9E2A44
# gpg: Good signature from "Alex Bennée (Master Work Key) <alex.bennee@linaro.org>" [full]
# Primary key fingerprint: 6685 AE99 E751 67BC AFC8  DF35 FBD0 DB09 5A9E 2A44

* remotes/stsquad/tags/pull-testing-060121-4:
  cirrus: don't run full qtest on macOS
  tests/acceptance: bound the size of readline in s390_ccw_virtio
  tests/iotests: drop test 312 from auto group
  tests/docker: Include 'ccache' in Debian base image

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>