tests/functional: Convert the riscv_opensbi avocado test into a standalone test

tests/functional: Convert the riscv_opensbi avocado test into a standalone test

The avocado test defined test functions for both, riscv32 and riscv64.
Since we can run the whole file with multiple targets in the new
framework, we can now consolidate the functions so we have to only
define one function per machine now.

Message-ID: <20240821082748.65853-23-thuth@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>