autocomplete test [nfc]: Use MarkedTextParse as the return type of parseMarkedText

This was first added in 0886948, but seems to have been
accidentally removed in 046ceab.

Author: sm-sayedi

test/model/autocomplete_test.dart

@@ -30,7 +30,7 @@ typedef MarkedTextParse = ({int? expectedSyntaxStart, TextEditingValue value});
 final zulipLocalizations = GlobalLocalizations.zulipLocalizations;
 
 void main() {
-  ({int? expectedSyntaxStart, TextEditingValue value}) parseMarkedText(String markedText) {
+  MarkedTextParse parseMarkedText(String markedText) {
     final TextSelection selection;
     int? expectedSyntaxStart;
     final textBuffer = StringBuffer();