tests/boot_linux_console: Use Avocado archive::gzip_uncompress()

tests/boot_linux_console: Use Avocado archive::gzip_uncompress()

Avocado 67.0 [*] introduced the avocado.utils.archive module which
provides handling of gzip files. Use the gzip_uncompress() method.

[*] https://avocado-framework.readthedocs.io/en/67.0/api/utils/avocado.utils.html#avocado.utils.archive.gzip_uncompress

Suggested-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20191028073441.6448-20-philmd@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>