I haven't bothered to ask for testing because shellfish isn't a big group of foods (in terms of things to avoid for a child dd's age).
At the moment, we don't buy shellfish so we have no problem avoiding it and they don't get it at school (only white fish or salmon).
I don't know how old your dd is. I would be more likely to push for testing if she was at the age where she had some independence (so was going out for meals or take aways without you being there).
I think it also depends how much you feel you need to know. Dd went through the whole spiel of allergy testing when she was a baby (she had egg, milk and soya allergy - they tested for those plus white fish, chicken, can't remember the others lol but not shellfish).
I always have Piriton on hand at home anyway (due to other allergies in the family) so I would suggest you do the same.
If it has frightened you and you want to know the answer, then you must push for the referral and the testing.
What I will do at some stage is rub a prawn/shellfish on the inside of dd's wrist and see if a rash comes up. For us, if that happens, I know dd is allergic to shellfish however I wouldn't recommend that course of action if you don't know your own dd's reactions/responses to allergens. We have had years of them so I know what signs to look out for in terms of deterioration etc.!
The GP response is quite typical. The treatment/testing of allergies moves on year by year and you'll find that even consultants give out different advice (you can see that by scanning the allergy boards!) so it's a real inexact science and a moving target in many ways. However, you have every right to push your GP, especially if you have not had experience of food allergies.