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.