[sandbox] Do not use QFileEngine

QFileEngine and related APIs are made private in Qt5. After this commit,
mapping file paths into sandbox is explicit -- code must be written to
support it.
[sandbox] Rename SSU_TEST_SANDBOX env variable

Use the name SSU_SANDBOX_PATH as introduced by 34f3c3c.
[sandbox] Re-enable sandbox dependent code

Reverts parts of 1cf47e0a62af9788edc0d74286af63043925bf1f
[ssuks] Use Sandbox::map() to map paths into sandbox
[sandbox] Allow to addWorldFiles() recursively
[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