--===== This is your original test table with just a bit of reformatting.
     -- I'd recommend that you change DECIMAL(15,0) to INT or BIGINT.
 CREATE TABLE #Test_Table
        (
        Test_key     DECIMAL(15,0),
        Question_Key DECIMAL(15,0),
        Result       VARCHAR(50)
        )
 INSERT INTO #Test_Table
        (Test_Key, Question_Key, Result)
 SELECT 12345, 567, 'abc' UNION ALL
 SELECT 12345, 234, 'xyz' UNION ALL
 SELECT 12345, 567, 'def' UNION ALL
 SELECT 12345, 567, 'ghi' UNION ALL
 SELECT 12345, 234, 'wxy' UNION ALL
 SELECT 12345, 123, 'lmn' UNION ALL
 SELECT 12345, 789, 'tuv' UNION ALL
 SELECT 12345, 123, 'lmn' UNION ALL
 SELECT 12345, 567, 'efg'
GO
--===== Assuming that the original table cannot be changed,
     -- create and populate a new table on the fly.  The
     -- IDENTITY column will reflect the desired sort order
     -- for creating a "Sequence" column.  ROW_NUMBER() or
     -- "RANK" in SQL Server 2005 would make this a lot easier.
 SELECT IDENTITY(INT,1,1) AS RowNum,
        Test_Key,
        Question_Key,
        CAST(0 AS INT)    AS Sequence,
        Result       
   INTO #MyWork
   FROM #Test_Table
  ORDER BY Test_Key, Question_Key, Result DESC
 
--===== Adding this clustered index is critical to getting
     -- the ensuing UPDATE to do things in the correct order.
     -- It MUST be the clustered index to work correctly.
  ALTER TABLE #MyWork
    ADD PRIMARY KEY CLUSTERED (RowNum)
   WITH FILLFACTOR = 100
 
--===== Declare a couple of obvious named variables...
DECLARE @Prev_Test_Key     INT,
        @Prev_Question_Key INT,
        @Prev_Sequence     INT
 
--===== Do the proprietary 3 part update.
     -- This works just as if you did it in a loop because,
     -- behind the scenes, and UPDATE IS a loop except it's
     -- ten's of times faster than a declared loop.
 UPDATE #MyWork
    SET @Prev_Sequence = Sequence = CASE
                                    WHEN @Prev_Test_Key = Test_Key
                                     AND @Prev_Question_Key = Question_Key
                                    THEN @Prev_Sequence + 1
                                    ELSE 1
                                     END,
       @Prev_Test_Key     = Test_Key,
       @Prev_Question_Key = Question_Key
  FROM #MyWork WITH (TABLOCKX)
OPTION (MAXDOP 1)
 
--===== Run the cross tab query to use the new Sequence column
 SELECT Test_Key,
        MAX(CASE WHEN Question_Key = 567 THEN Result END) AS [First],
        MAX(CASE WHEN Question_Key = 234 THEN Result END) AS [Second],
        MAX(CASE WHEN Question_Key = 123 THEN Result END) AS [Third],
        MAX(CASE WHEN Question_Key = 789 THEN Result END) AS [Fourth]
   FROM #MyWork
  GROUP BY Test_Key, Sequence