tests/functional: Split the test_aarch64_sbsaref test

tests/functional: Split the test_aarch64_sbsaref test

The test_aarch64_sbsaref test is the test with the longest runtime
in our functional test suite. Split it into parts so that it can
be run on multiple CPUs in parallel.

For this we have to move the fetch_firmware() function out of the
class definition to be able to reuse it easily from the other tests
(deriving the Aarch64SbsarefAlpine and Aarch64SbsarefFreeBSD directly
from Aarch64SbsarefMachine does not work, unfortunately, since we'd
inherit the test_sbsaref_edk2_firmware() function that way, causing
it to be run multiple times - and keeping the fetch_firmware() in
a separate class without the test_sbsaref_edk2_firmware() function
also does not work since the "make precache-functional" won't work
in that case ==> turning fetch_firmware() into a static function is
the best option).

Message-ID: <20241106175029.1000589-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>