Fixed BigInt multiplication test cases

tests/UBigIntTestCases.pas

@@ -628,7 +628,7 @@ begin
   s := TBigInt.FromHexadecimalString(AHexValueProduct);
   AssertTrue('Product of BigInt from ''' + AHexValueLeft + ''' and from ''' + AHexValueRight
     + ''' was not equal to the BigInt from ''' + AHexValueProduct + '''.',
-    s = a + b);
+    s = a * b);
 end;
 
 procedure TBigIntProductTestCase.TestShort;