Improve `AttachedRoutes` conformance test

[mlavacca]

What would you like to be added:

At the moment, the test cases related to the AttachedRoutes are structured as follows:

• the first one has one Gateway with one listener and one HTTPRoute referring to it as a parent. The test verifies that the status.listeners[0].AttachedRoutes field gets the value 1 assigned.
• the second one has one Gateway with one listener and two HTTPRoutes referring to it as a parent. The test verifies that the status.listeners[0].AttachedRoutes field gets the value 2 assigned.

Alongside the above test cases, we should verify that a gateway with many listeners has the corresponding AttachedRoutes field assigned only if the Routes can really attach to them.

Example:

• create a Gateway with two listeners using two different protocols: HTTP and UDP
• create an HTTPRoute
• only the HTTP listener should have the AttachedRoutes field to 1, the UDP listener value should be 0.

[levikobi]

Can I help with this one?

[shaneutt]

@levikobi we would love to have you help out, and appreciate the offer!

Do you have an implementation in mind that you're already contributing to which runs the conformance test suite?

[levikobi]

Thanks @shaneutt
I can use envoy-gateway if that's ok. I'm not a contributor though.

[mlavacca]

Hi @levikobi! We'd definitely appreciate your contribution! You can use whichever Gateway implementation you prefer, envoy is 👍!

[shaneutt]

Sounds good, let us know if you need any support (by posting comments here, or checking in #sig-network-gateway-api on slack)!

/assign @levikobi