finally get r01490.vtc right

(i hope)

Trouble here is that in pool_herder(), we access pp->dry unprotected, so
we might see an old value, thus we might breed more than wthread_min
even if the dry condition does not exist any more.

So for the vtc, we need to wait until wthread_timeout has passed and the
surplus thread has been kissed to death.

Notice that this does not change with #2942 because there the same
unprotected access happens to lqueue.

bin/varnishtest/tests/r01490.vtc

@@ -13,7 +13,8 @@ varnish v1 \
 	-vcl+backend {}
 varnish v1 -start
 
-delay 2
+# we might have over-bred
+delay 11
 
 varnish v1 -expect threads == 2