MXS-2900 Move testcore library files to a dedicated directory

The library is now named "maxtest". The related include-files are, for
now, usable without designating the full include path. This may be changed
later, but would require modifying every test.

maxscale-system-test/mysqlmon_failover_auto.cpp

@@ -11,7 +11,7 @@
  * Public License.
  */
 
-#include "testconnections.h"
+#include "mariadb_nodes.h"
 #include "failover_common.cpp"
 
 int main(int argc, char** argv)