One of the clearest indices of Bangladesh shedding the minnow tag is in how they are referenced when they aren't directly the subject of the conversation. It would be a mistake to assume that international cricket exists in two binary states: world beater or minnow. There exists a vast space in between filled with teams unlikely to be crowned champions, but also able to compete with any team on a given day (ie middling sides like New Zealand). As a few people have suggested, winning Test matches in Australia/England/South Africa/India will of course signify entry into the topmost bracket, but there are more useful measures. Such as what those knowledgable within the game say as a matter of fact.
Here is an article on Younis Khan's possible non-retirement and an aside on Bangladesh...the type of comment that even a year ago would not be made:
Given his experience and recent Test form, and given that a tour to Bangladesh this summer will provide a far sterner test of their batting than tours past, a case can be readily made for wanting Younis to stay on.