Espérons que je réussisse mon test demain = Hopefully, I will pass my test tomorrow. It's idiomatic . In this context you can conjugate at the 1st person plural in the imperative + subjunctive. You could also use the 1st singular présent in the indicative + future."J'espère que je réussirai mon test demain", It's a matter of tenses concordance. " Passer un examen" is a false friend, it means to attempt an exam. Réussir un examen = to pass an exam. "On l'esperons" is wrong, it should be "on l'espère" or "nous l'espérons". Hopefully, you understand this explanation. If not, drop me a line.

espérons = littlerally "hope! (in the plural form)" but it means "let's hope". hopefully = litterally "avec espoir" but in English, hopefully is used to emphasize how much you want something, and how you want it to run smoothly, and in that case it's harder to translate. That's why I suggest you "avec un peu de chance" (with a bit of luck) & "si tout va bien" (if everything goes according to plan) "croisons les doigts" (cross fingers) That said, I prefer "espérons QUE je passe ce test demain" in your example. (Hope THAT I pass this test - that's how we phrase it)

hopefully = on l’espérons..................................."j’espère que je passerais mon test demain" is more coherent .........................................