Reduce default thread pool size to 10 threads, no need to stress

the pthread implementation in *every* test case.

bin/varnishtest/vtc_varnish.c

@@ -386,6 +386,7 @@ varnish_launch(struct varnish *v)
 	VSB_printf(vsb, " -p auto_restart=off");
 	VSB_printf(vsb, " -p syslog_cli_traffic=off");
 	VSB_printf(vsb, " -p sigsegv_handler=on");
+	VSB_printf(vsb, " -p thread_pool_min=10");
 	VSB_printf(vsb, " -a '%s'", "127.0.0.1:0");
 	VSB_printf(vsb, " -M '%s %s'", abuf, pbuf);
 	VSB_printf(vsb, " -P %s/varnishd.pid", v->workdir);