Randomize --connect-to if multiple matching options

[thomasgl-orange]

Here is a PR to better handle this:

oha \
  --connect-to something.com:443:backend1.fqdn:8443 \
  --connect-to something.com:443:backend2.fqdn:8443 \
  https://something.com

In this case, a random choice will be made for each request between backend1.fqdn:8443 and backend2.fqdn:8443. Where as currently, while the syntax is not rejected, it's always the same target which would be picked.

My use case is being able to bypass an actual TCP load-balancer in front of the backend servers, just to check it doesn't have a significant impact on the overall performances.

I've seen that DNS resolution already makes some similar random choice per-request in case several IP are returned for the domain. I guess I could use that (with some local dnsmasq entries) for my use case, but this --connect-to approach sounds more intuitive.

The second commit is a test case with 100 requests to spread across two servers, listening on two local ports. It checks that both receive at least some requests, and that the total number of requests received is indeed 100.

[hatoo]

Please fix clippy warning.
Others are good!

[thomasgl-orange]

Thanks @hatoo for the quick review and reply, much appreciated!
I've just pushed 1bb9e1b to fix the clippy warning.

[hatoo]

thank you!