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/mysqlmon_failover_stress.cpp

@@ -227,7 +227,7 @@ private:
         int id = s_nRows * random_decimal_fraction();
 
         ss_dassert(id >= 0);
-        ss_dassert(id <= s_nRows);
+        ss_dassert(id <= (int)s_nRows);
 
         return id;
     }
@@ -503,8 +503,6 @@ void check_server_statuses(TestConnections& test)
 
 void run(TestConnections& test)
 {
-    int n_threads = Client::DEFAULT_N_CLIENTS;
-
     cout << "\nConnecting to MaxScale." << endl;
     test.maxscales->connect_maxscale();