simplify tests confusions with mux usage, remove Mux accessor

[umputun]

The presence of the Bundle.Mux accessor seems largely superfluous and potentially confusing. Moreover, it reveals excessive internal details, thereby hindering modifications. For instance, it obstructs shifts in the core strategy, such as applying middleware not to each registered route individually, but to groups or subgroups of routes.

Also adjusted tests to use the group as a handler, not the underlying mux