tests: acpi: print ASL diff in verbose mode

print ASL difference if there is any when
executing 'make V=1 check'.
Use 'DIFF' environment variable to determine
which diff utility to use and if it's not set
notify user by printing warning that DIFF is
not set if run in verbose mode and there is
difference in ASL.

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


