tests/functional: Add base classes for the upcoming pytest-based tests

tests/functional: Add base classes for the upcoming pytest-based tests

The files are mostly a copy of the tests/avocado/avocado_qemu/__init__.py
file with some adjustments to get rid of the Avocado dependencies (i.e.
we also have to drop the LinuxSSHMixIn and LinuxTest for now).

The emulator binary and build directory are now passed via
environment variables that will be set via meson.build later.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20240830133841.142644-9-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>