tests/functional: Add QemuUserTest class

tests/functional: Add QemuUserTest class

Per commit 5334df4822 ("tests/avocado: Introduce
QemuUserTest base class"):

  Similarly to the 'System' Test base class with methods
  for testing system emulation, the QemuUserTest class
  contains methods useful to test user-mode emulation.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240822104238.75045-2-philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20240830133841.142644-38-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>