It is possible, also its very easy, but testing is offline for ages now. so there is no need for it right?
Use yogda. decompile prod haacks change realmofthemadgod.appspot.comⓘ to testing****tmg.appspot.comⓘ something like that haavent doone it in a while
1) go to testing.realmofthemadgod.com/version.txtⓘ 2) go to testing.realmofthemadgod.com/AssembleeGameClient(versionⓘ here) 3) Save it 4) download ModSelector 5) switch your .swf to ModSelector's one 6) Select mods you want but don't select Connect to Production Server PROFIT