test: add tests of graylog_stream_rule

[suzuki-shunsuke]

No description provided.

[codecov]

Codecov Report

Merging #226 into master will increase coverage by 0.5%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #226     +/-   ##
=========================================
+ Coverage   52.33%   52.83%   +0.5%     
=========================================
  Files         125      125             
  Lines        5962     5962             
=========================================
+ Hits         3120     3150     +30     
+ Misses       2665     2620     -45     
- Partials      177      192     +15

Impacted Files	Coverage Δ
terraform/graylog/resource_stream_rule.go	68.75% <0%> (+31.25%)	⬆️

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