Take -Wall into use for tests

Take almost all warnings into use except `format-overflow` and
`unused-function`. format-overflow needs to be handled in a separate
commit.

maxscale-system-test/mariadb_nodes.h

@@ -38,7 +38,7 @@ public:
      */
     Mariadb_nodes(const char *pref, const char *test_cwd, bool verbose);
 
-    ~Mariadb_nodes();
+    virtual ~Mariadb_nodes();
 
     /**
     * @brief  MYSQL structs for every backend node