Wait for Route readiness in TestSingleConcurrency

[bbrowning]

TestSingleConcurrency was waiting for the Revision to be Ready but should also wait on the Route to both be Ready and respond successfully to a request before proceeding with the rest of the test.

This pulls probeDomain from the blue green test out into test.ProbeDomain because, until #1582 is fixed, every test that sends traffic to a Route should probably probe the domain first in some way to prevent flakes.

Fixes #2306

Release Note

NONE

[knative-prow-robot]

@bbrowning: 1 warning.

In response to this:

TestSingleConcurrency was waiting for the Revision to be Ready but should also wait on the Route to both be Ready and respond successfully to a request before proceeding with the rest of the test.

This pulls probeDomain from the blue green test out into test.ProbeDomain because, until #1582 is fixed, every test that sends traffic to a Route should probably probe the domain first in some way to prevent flakes.

Fixes #2306

Release Note

NONE

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[tcnghia]

/lgtm

[tcnghia]

/retest

[bbrowning]

/test pull-knative-serving-integration-tests

[mattmoor]

/approve

[knative-prow-robot]

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This pull-request has been approved by: bbrowning, mattmoor

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