MXS-2900 Include from predefined search directories in maxtest

Similar style is used in other MaxScale libraries.

maxscale-system-test/maxtest/src/execute_cmd.cc

@@ -1,9 +1,8 @@
 #include <iostream>
-#include <unistd.h>
 #include <stdio.h>
 #include <string.h>
 #include <errno.h>
-#include "execute_cmd.hh"
+#include <maxtest/execute_cmd.hh>
 
 using namespace std;