Cluster Autoscaler: Fix vet failures with -vet=all

The copylocks, composites, and lostcancel checks were not part of the
prior set of checks. The following fixes are needed to pass the checks
now that they are enabled.

- equinixmetal: copylocks - store *equinixMetalNodeGroup in slice
  to avoid copying sync.Mutex
- magnum/fixtures: composites - use keyed fields for Instance,
  InstanceStatus, and InstanceErrorInfo struct literals
- oci/instancepools: lostcancel - capture and defer cancel func
  from context.WithTimeout
- config/flags tests: composites - use keyed fields for
  ShutdownGracePeriodByPodPriority struct literals
- core tests: composites - use keyed fields for
  MockAutoprovisioningNodeGroupManager and
  MockAutoprovisioningNodeGroupListProcessor struct literals
- grpcplugin tests: copylocks - use index-based range to avoid copying
  protobuf MessageState containing sync.Mutex