Add flexibility to the path of the script

[yorugac]

Currently, k6-operator with ConfigMap and VolumeClaim options expects JS script to be present in /test folder: https://github.com/grafana/k6-operator/blob/de5794766160bca70dfb8a06e6868b0ae8aa2c86/pkg/types/types.go#L22-L57

These two options don't match how LocalFile is implemented and it also causes issues during usage.

Caveat: in order not to break anyone's existing setup, it'd be good to keep backwards-compatibility with the current approach.