test: add unit tests for docsettings

frontend/docsettings.lua

@@ -2,6 +2,7 @@ local lfs = require("libs/libkoreader-lfs")
 local DataStorage = require("datastorage")
 local dump = require("dump")
 local purgeDir = require("ffi/util").purgeDir
+local logger = require("logger")
 
 local DocSettings = {}
 
@@ -24,6 +25,7 @@ function DocSettings:getSidecarDir(doc_path)
     end
     -- We shouldn't be called with anything but files with registered
     -- extensions, but in case we are, return something useful
+    logger.err("getSidecarFile called with unexpected path:", doc_path)
     return doc_path..".sdr"
 end