tests/avocado: Move LinuxTest related code into a separate file

Only some few tests are using the LinuxTest class. Move the related
code into a separate file so that this does not pollute the main
namespace.

Message-ID: <20240719095031.32814-1-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
8 files changed