Changed location of unit socket in unit tests

Changed location of unit socket in unit tests

Previously it was stored in TEST_DATA_BUILDDIR which resides inside repo sources.
Due to the fact that when repo was located in deep patch creating socket was
failing as max socket patch is about 100 characters. Now it is located
in temp folder managed by Sandbox class. The sandbox directory is created
in test constructor and deleted in destructor. As the temp directory
is in form /tmp/kea-XXXXXX the lnegth is always lower than 1000, so running
the unit tests never fails.