Add visibility test for Istio

[nak3]

This patch adds visbility test for istio.

Also, it changes HTTPRoute's Gateway selector to GatewayAllowAll
from GatewayAllowFromList. (So #47 is also covered eventually.)

/cc @ZhiminXiang @markusthoemmes @howardjohn

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In response to this:

This patch adds visbility test for istio.

Also, it changes HTTPRoute's Gateway selector to GatewayAllowAll
from GatewayAllowFromList. (So #47 is also covered eventually.)

/cc @ZhiminXiang @markusthoemmes @howardjohn

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[codecov]

Codecov Report

Merging #74 (c9e256c) into main (4ef7bba) will not change coverage.
The diff coverage is 0.00%.

@@          Coverage Diff          @@
##            main     #74   +/-   ##
=====================================
  Coverage   0.52%   0.52%           
=====================================
  Files         13      13           
  Lines       1713    1713           
=====================================
  Hits           9       9           
  Misses      1704    1704           

Impacted Files	Coverage Δ
test/conformance/ingressv2/run.go	0.00% <ø> (ø)
test/conformance/ingressv2/util.go	1.30% <0.00%> (ø)
test/conformance/ingressv2/visibility.go	0.00% <0.00%> (ø)

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[howardjohn]

How come we aren't using GatewayAllowFromList anymore?

[nak3]

Since Istio's Gateway needs to be deployed in istio-system namespace, Contour also needs to use the Gateway in istio-system namespace if we use GatewayAllowFromList. (That's so strange.)

I will fix it by #75 in a separate PR.

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[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: howardjohn, nak3

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[ZhiminXiang]

/lgtm