fix(controller): TestPodExists test, add delay. Fixes #6458

[tczhao]

Fixes #6458

There's a slight delay between creating pod and informer getting this event,
which result in flaky test

Tested using

go test -count=100 -run=TestPodExists ./workflow/controller

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[codecov]

Codecov Report

Merging #6783 (5513ea2) into master (63aecdb) will decrease coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 5513ea2 differs from pull request most recent head 459a4d8. Consider uploading reports for the commit 459a4d8 to get more accurate results

@@            Coverage Diff             @@
##           master    #6783      +/-   ##
==========================================
- Coverage   48.65%   48.63%   -0.03%     
==========================================
  Files         264      264              
  Lines       19076    19076              
==========================================
- Hits         9282     9278       -4     
- Misses       8749     8752       +3     
- Partials     1045     1046       +1     

Impacted Files	Coverage Δ
cmd/argoexec/commands/emissary.go	50.35% <0.00%> (-1.44%)	⬇️
cmd/argo/commands/get.go	58.89% <0.00%> (-0.30%)	⬇️
workflow/controller/operator.go	71.06% <0.00%> (-0.15%)	⬇️
workflow/metrics/server.go	19.29% <0.00%> (+3.50%)	⬆️

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