Remove allactive/default testmods directory

[billsacks]

As @fischer-ncar and I noticed, the allactive/default testmods directory just contains a bunch of empty files. We should remove this testmods directory to avoid confusion. Then we'll need to (1) change tests that use allactive/default to instead have no testmod, and (2) remove references to this from include_user_mods files in other allactive testmods.

@fischer-ncar - #84 started with the point of doing this, but then I realized that it would change test names and so break some baseline comparisons, so I thought it would be best to leave this to you to do at a convenient time.

[billsacks]

@fischer-ncar just going through old issues and noticed this one. This seems like a relatively easy change to make at some point.