Don't update survey prompt if survey prompt is not shown to stdout

[tejal29]

relates to #6166

While looking in to code on when skaffold updates prompt show, i discovered a bug.

1. Made sure the test failed when we do update the timestamp when std out is not set.

/private/var/folders/gk/s778hvkj0lb0zl95ywvw0snr00cfc0/T/___TestDisplaySurveyForm_in_github_com_GoogleContainerTools_skaffold_pkg_skaffold_survey -test.v -test.paniconexit0 -test.run ^\QTestDisplaySurveyForm\E$
=== RUN   TestDisplaySurveyForm
=== RUN   TestDisplaySurveyForm/std_out
=== RUN   TestDisplaySurveyForm/not_std_out
    survey_test.go:61: unexpected error: not expected to call
--- FAIL: TestDisplaySurveyForm (0.00s)
    --- PASS: TestDisplaySurveyForm/std_out (0.00s)
    --- FAIL: TestDisplaySurveyForm/not_std_out (0.00s)

FAIL

Process finished with exit code 1

[codecov]

Codecov Report

Merging #6192 (6c9ca99) into master (db90d38) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #6192   +/-   ##
=======================================
  Coverage   71.08%   71.08%           
=======================================
  Files         485      485           
  Lines       21603    21604    +1     
=======================================
+ Hits        15356    15357    +1     
  Misses       5264     5264           
  Partials      983      983           

Impacted Files	Coverage Δ
pkg/skaffold/survey/survey.go	58.06% <100.00%> (+1.39%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update db90d38...6c9ca99. Read the comment docs.

[briandealwis]

Minor tweak: mockStdOut should go above expected since it's a test input