Fix broken UTF subString: tests
Fix broken UTF subString: tests

The old Character>>_separateSubStringsIn: did not work correctly
if the receiver required multiple code units to represent in the encoding
of the argument String. This had been fixed in split: after the subStrings:
change was made.

objectarts/Dolphin,jgfoster/Dolphin,dolphinsmalltalk/Dolphin,dolphinsmalltalk/Dolphin,jgfoster/Dolphin,shoshanatech/Dolphin,jgfoster/Dolphin,shoshanatech/Dolphin,dolphinsmalltalk/Dolphin,objectarts/Dolphin,dolphinsmalltalk/Dolphin,shoshanatech/Dolphin,jgfoster/Dolphin,shoshanatech/Dolphin