DRY up the retry methods in monitoring tests

[Temikus]

Drive-by fix for an old TODO in monitoring tests.

[codecov]

Codecov Report

Merging #501 into master will increase coverage by 1.51%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #501      +/-   ##
==========================================
+ Coverage   87.50%   89.01%   +1.51%     
==========================================
  Files         341      341              
  Lines        5736     5736              
==========================================
+ Hits         5019     5106      +87     
+ Misses        717      630      -87     

Impacted Files	Coverage Δ
.../fog/compute/google/requests/get_instance_group.rb	100.00% <0.00%> (+12.50%)	⬆️
...og/compute/google/requests/list_instance_groups.rb	100.00% <0.00%> (+12.50%)	⬆️
...g/compute/google/requests/delete_instance_group.rb	100.00% <0.00%> (+12.50%)	⬆️
...g/compute/google/requests/get_instance_template.rb	100.00% <0.00%> (+12.50%)	⬆️
...compute/google/requests/list_instance_templates.rb	100.00% <0.00%> (+12.50%)	⬆️
...ompute/google/requests/delete_instance_template.rb	100.00% <0.00%> (+12.50%)	⬆️
...pute/google/requests/get_instance_group_manager.rb	100.00% <0.00%> (+12.50%)	⬆️
...te/google/requests/list_instance_group_managers.rb	100.00% <0.00%> (+12.50%)	⬆️
...e/google/requests/delete_instance_group_manager.rb	100.00% <0.00%> (+12.50%)	⬆️
...google/requests/list_aggregated_instance_groups.rb	100.00% <0.00%> (+12.50%)	⬆️
... and 10 more

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