* Removed obsolete apkindex_files cache testcase (the corresponding
function has been removed in #345 already).
* Fix test_challenge_apk: It failed on Travis, because we're accessing
/etc/abuild.conf, which only exists after initializing the build environment.
It's a random dummy file anyway, so I've replaced it with another file.
* Fix test_folder_size: accept a tolerance in the result
ed94ab7449