Continuous Integration and Isolated Feature Testing
First of all, git is certainly not to blame for merges taking a long time. Clean merging is the hallmark of git, unless it is used in some broken way. So I encourage you to look at what exactly seems to be the underlying problem. If your devs regularly make sweeping changes to the same files, then it's likely more a problem of cohesion/coupling than of the underlying merge tool.

Second; I like to do isolated feature-testing, if by that you mean that your test suite is ran against the feature branch (with only the branch changes) in a clean environment. In fact, I like to test all branches in this way. How could the develop or master branch become green if one of the features going into it are not? It goes without saying that develop / master need to be tested as well. 

You could even go so far as to do a pre-merge test by doing a merge of feature into temp_develop_with_feature (or whatever you may call it), that is, simulate the merging of a branch into develop (under a different name) and test that. 

Obviously, all of this assumes that sufficient testing ressources (VMs etc.) are available and fast enough, and that everything is 100% automated, both of which should be rather easy to achieve with something like Jenkins, Gitlab or whatever tool you use (if not, then it's maybe time to look at another one...).