Fix autopkgtest for OpenLDAP 2.5.5

We're working towards getting OpenLDAP 2.5.5 uploaded to unstable, and nsscache's autopkgtest fails because it uses the now-removed BDB backend. This MR changes the upstream test to use the MDB backend.

I've checked and the test is passing, but I'm not entirely sure that this is the right way to go. It doesn't seem like nsscache has any code to handle MDB (but I did find code to handle BDB), so maybe the project itself needs to be updated in this case? Anyway, I'd like your input before I go ahead and suggest this upstream. Also, @rtandy's input is very welcome, as usual.

Thanks.