Integration testing patches

- Rename tests/acceptance/ -> tests/avocado/
- Rename avocado_qemu.Test -> avocado_qemu.QemuSystemTest
- Introduce QemuUserTest class
- Add the first linux-user test, covering the bFLT loader

# gpg: Signature made Mon 08 Nov 2021 05:16:46 PM CET
# gpg:                using RSA key FAABE75E12917221DCFD6BB2E3E32C2CDEADC0DE
# gpg: Good signature from "Philippe Mathieu-Daudé (F4BUG) <f4bug@amsat.org>" [full]

* remotes/philmd/tags/avocado-20211108:
  tests/avocado: Remove p7zip binary availability check
  tests/avocado: Rename avocado_qemu.Test -> QemuSystemTest
  tests/avocado: Add bFLT loader linux-user test
  tests/avocado: Share useful helpers from virtiofs_submounts test
  tests/avocado: Introduce QemuUserTest base class
  tests/avocado: Make pick_default_qemu_bin() more generic
  tests/avocado: Extract QemuBaseTest from Test
  tests/acceptance: rename tests acceptance to tests avocado
  tests/acceptance: introduce new check-avocado target

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>