kubernetes: Use errors.Is in PluginArgs validation tests

/sig scheduling /kind cleanup

Files:

• https://github.com/kubernetes/kubernetes/blob/master/pkg/scheduler/apis/config/validation/validation_test.go
• https://github.com/kubernetes/kubernetes/blob/master/pkg/scheduler/apis/config/validation/validation_pluginargs_test.go

We currently compare the error strings instead of leveraging errors.Is, which will ensure the correct types are used.