Improve test coverage

Improve test coverage for `Process()` method of `kubetmuxp.Config`.

[https://github.com/arunvelsriram/kube-tmuxp/blob/a668683956ccb8fe1db966eabf365127f2d5cff0/pkg/kubetmuxp/kubetmuxp.go#L108](https://github.com/arunvelsriram/kube-tmuxp/blob/a668683956ccb8fe1db966eabf365127f2d5cff0/pkg/kubetmuxp/kubetmuxp.go#L108)