KVM: selftests: Extend x86's sync_regs_test to check for exception races

KVM: selftests: Extend x86's sync_regs_test to check for exception races

Attempt to set the to-be-queued exception to be both pending and injected
_after_ KVM_CAP_SYNC_REGS's kvm_vcpu_ioctl_x86_set_vcpu_events() squashes
the pending exception (if there's also an injected exception).  Buggy KVM
versions will eventually yell loudly about having impossible state when
processing queued excpetions, e.g.

  WARNING: CPU: 0 PID: 1115 at arch/x86/kvm/x86.c:10095 kvm_check_and_inject_events+0x220/0x500 [kvm]

  arch/x86/kvm/x86.c:kvm_check_and_inject_events():
  WARN_ON_ONCE(vcpu->arch.exception.injected &&
	       vcpu->arch.exception.pending);

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://lore.kernel.org/r/20230728001606.2275586-3-mhal@rbox.co
[sean: split to separate patch, massage changelog and comment]
Signed-off-by: Sean Christopherson <seanjc@google.com>