Skip flaky OpenCensus exporter test until fixed

[markusthoemmes]

Ref #1672

This seriously impacts developer productivity, so let's skip until fixed.

/assign @julz @vaikas

[julz]

👍️

/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[codecov]

Codecov Report

Merging #1870 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1870   +/-   ##
=======================================
  Coverage   69.33%   69.33%           
=======================================
  Files         223      223           
  Lines        9477     9477           
=======================================
  Hits         6571     6571           
  Misses       2574     2574           
  Partials      332      332           

Impacted Files	Coverage Δ
apis/duck/patch.go	100.00% <ø> (ø)

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