testobj3: fix test

Use RTL StringRefCount instead of local GetRefCount that only supports some
platforms and depends on the internal layout of ansistring headers

tests/test/units/objects/testobj3.pp

@@ -2,11 +2,6 @@ program testobj3;
 uses
   Objects;
 
-function GetRefCount(const S: RawByteString): SizeInt;
-begin
-  GetRefCount:=PSizeInt(PByte(S)-2*SizeOf(SizeInt))^;
-end;
-
 procedure Error(ErrNo: Integer);
 begin
   Writeln('Error! ', ErrNo);
@@ -25,21 +20,22 @@ begin
   S[3] := 'l';
   S[4] := 'l';
   S[5] := 'o';
-  if GetRefCount(S)<>1 then
+  writeln(stringrefcount(s));
+  if stringrefcount(S)<>1 then
     Error(1);
   coll := New(PRawByteStringCollection, Init(100, 100));
   coll^.AtInsert(0, S);
-  if GetRefCount(S)<>2 then
+  if stringrefcount(S)<>2 then
     Error(2);
   S2 := RawByteString(coll^.At(0));
-  if GetRefCount(S)<>3 then
+  if stringrefcount(S)<>3 then
     Error(3);
   if S2<>'Hello' then
     Error(4);
   if RawByteString(coll^.At(0))<>'Hello' then
     Error(5);
   Dispose(coll, Done);
-  if GetRefCount(S)<>2 then
+  if stringrefcount(S)<>2 then
     Error(6);
 end;