It's been a while since I flew Squirrels but from memory part of the aim of the test is to check that the horn will come on to warn of a loss of hyd pressure when you select "test". If you perform the hyd test at idle you will not be able to test this part of the system. Furthermore, after you select the isolate switch the horn is re-set to prove that it has now been isolated from the hyd system and will now be available to operate in the event of low NR.