Fix libprivilege-control tests

Fix libprivilege-control tests

[Problem] Libprivilege-control assigns the same label "User" to all
applications. Because of that some of tests make no sense. User id is fixed to
5000 but there's no such user in the system. Wrt-client binary does not exist
so getting its smack label fails.
[Solution] Meaningless asserts commented. User id retrieved from tzconfig. DAC
groups database cleared after every perm_app_set_privilege test.

[Verification] Run libprivilege-control-tests --output=text

Change-Id: Ib8e1f4deafd7033c7ac892af6afd9d46c724c062