tests: acpi: reuse fetch_table() for fetching FACS and DSDT

tests: acpi: reuse fetch_table() for fetching FACS and DSDT

It allows to remove a bit more of code duplication and
reuse common utility to get ACPI tables from guest (modulo RSDP).

While at it, consolidate signature checking into fetch_table() instead
of open-codding it.

Considering FACS is special and doesn't have checksum, make checksum
validation optin, the same goes for signature verification.

PS:
By pure accident, patch also fixes FACS not being tested against
reference table since it wasn't added to data::tables list.
But we managed not to regress it since reference file was added
by commit
   (d25979380 acpi unit-test: add test files)
back in 2013

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>