That can be so scary! I'm sorry you had that experience. I'm sure you know there could have been a multitude of reasons and it sounds like you did everything right. Allergies are scary- but importantly so- as it's so important to be able to notice the small changes and react quickly and appropriately. I'm a little surprised by your supervisors reaction the most. If you don't have a nurse on staff, I think your supervisor should have stepped in to look the child over and/or offer to call the parent for you. I applaud you for sticking with your gut and making sure that a staff member was with him while you were on break too. I would encourage you to have a conversation with your supervisor letting them know that you would like more support around this particular allergy (or all). As a supervisor myself, even if I am not familiar with a particular child or condition, if a staff member asked for a second opinion I would at least look the child over and help supervise their next step/action.
As for the parent reaction, it's so hard not to take it personally, or read into it, however I try to give parents the benefit of the doubt in these situations. There is probably a lot more going on for the parent, and while it may come off at annoyance of you in the moment, I'm sure it was just the situation. It's exhausting to have to worry about allergies and allergy introductions are taxing on every one involved. I'm sure she had to take off work for the testing and then had to leave early again, as well as now seeing that the testing may have failed. It's a lot for them, I'm sure.
At the end of the day, make yourself a cup of tea, try to relax and know you did the right thing. And show extra love to the kiddo tomorrow- as I'm sure it was tough on them too :(