Failing compilation of unit tests due to using namespace isc::db::test

The compilation error is as follows:

20:49:49.208  cfg_db_access_unittest.cc:31:26: error: ‘test’ is not a namespace-name
20:49:49.208   using namespace isc::db::test;
20:49:49.208                            ^~~~
20:49:49.208  cfg_db_access_unittest.cc:31:30: error: expected namespace-name before ‘;’ token
20:49:49.208   using namespace isc::db::test;
20:49:49.208                                ^
20:49:49.208  Makefile:1116: recipe for target 'libdhcpsrv_unittests-cfg_db_access_unittest.o' failed

It is related to the conditional inclusion of the header files providing this namespace. When MYSQL, Postgres etc are not in use the header files are not included and therefore the namespace is missing.