tests/cpu-policy: Confirm that CPUID serialisation is sorted

tests/cpu-policy: Confirm that CPUID serialisation is sorted

The existing x86_cpuid_copy_to_buffer() does produce sorted results, and we're
about to start relying on this.  Extend the unit tests.

As test_cpuid_serialise_success() is a fairly limited set of synthetic
examples right now, introduce test_cpuid_current() to operate on the full
policy for the current CPU.

Tweak the fail() macro to allow for simplified control flow.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Paul Durrant <paul@xen.org>