Use libosmocore's DLOAP logging category for OAP.
oap_client_test.c: make sure DLOAP is in DEBUG level to not lose any logging
messages from experr.
Todo: we're using a "Library" logging category, which is not really what the
library category was intended for. Instead, the OAP client should probably be
given a logging category like DVLR or DGPRS in its initialization API.
Related: OS#1592
Change-Id: Ic765c19381b0d983da90a5d8aee9cd17e31cf34a
Neels Hofmeyr