samples(test): add retry rule in tests

[anguillanneuf]

Fixes #633

[codecov]

Codecov Report

Merging #645 (997e851) into master (d2e5ab6) will increase coverage by 0.52%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #645      +/-   ##
============================================
+ Coverage     78.30%   78.83%   +0.52%     
  Complexity      379      379              
============================================
  Files            27       27              
  Lines          3416     3420       +4     
  Branches        148      171      +23     
============================================
+ Hits           2675     2696      +21     
- Misses          649      651       +2     
+ Partials         92       73      -19     

Impacted Files	Coverage Δ	Complexity Δ
...cloud/pubsub/v1/StreamingSubscriberConnection.java	67.30% <0.00%> (-0.46%)	15.00% <0.00%> (ø%)
...a/com/google/cloud/pubsub/v1/TopicAdminClient.java	58.95% <0.00%> (+2.31%)	34.00% <0.00%> (ø%)
...om/google/cloud/pubsub/v1/SchemaServiceClient.java	60.91% <0.00%> (+3.44%)	20.00% <0.00%> (ø%)
...oogle/cloud/pubsub/v1/SubscriptionAdminClient.java	63.06% <0.00%> (+5.40%)	52.00% <0.00%> (ø%)

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