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_rejoin_bad2.cpp

@@ -66,7 +66,6 @@ static void expect(TestConnections& test, const char* zServer, const char* zStat
 
 int main(int argc, char** argv)
 {
-    char result_tmp[bufsize];
     interactive = strcmp(argv[argc - 1], "interactive") == 0;
     Mariadb_nodes::require_gtid(true);
     TestConnections test(argc, argv);