we need more threads

we run into the problem solved by
https://github.com/varnishcache/varnish-cache/pull/2942

purely for convenience, we send pass'ed backend requests to ourself (in
order not to have to fiddle with the number of requests on servers), yet
it can happen that all threads are used up and the herder fails to breed
more.

So the solution is to ensure that we do not run into any dependency on
the herder.

this stabilizes permutations.vtc for -j 40 -n 1000

src/tests/permutations.vtc

@@ -42,6 +42,7 @@ varnish v1 -arg "-a 127.0.0.1:0 -a self=${tmpdir}/v1.sock" \
 	-arg "-p first_byte_timeout=5" \
 	-arg "-p between_bytes_timeout=5" \
 	-arg "-p backend_idle_timeout=1" \
+	-arg "-p thread_pool_min=128" \
 	-vcl+backend {
 	import ${vmod_pesi};