getfield implementation splits only on character and not string

[axel3rd]

As documented for getfield, it splits a string on a character or string.

Currently the TesTcl implementation does the split only on character because split is only on chars:

https://github.com/landro/TesTcl/blob/bd55ca92353b08cd02782458af378f074243984a/src/global.tcl#L123-L127