add test for things `fdroid build` cleans
add test for things `fdroid build` cleans

This needs a lot of mocking because build.build_local() is a
gianormous single function.

fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server