[ssuks] Make use of Sandbox::addWorldFiles()
[tests] Do not export SSU_SANDBOX_DIR manually

Somehow I forgot to refactorize this in past. Available since 8b12419.
[tests] Update ut_ssuurlresolvertest
Add qt-testlib to build requirements
Use dir.filePath() for generating pathnames
Merge branch 'master' of github.com:nemomobile/ssu
[sandbox] Create missing dirs with addWorldFiles()
[sandbox] Implement in-process directory copy
[sandbox] Implement in-process tmp dir creation