test: add tests of graylog_stream

[suzuki-shunsuke]

No description provided.

[codecov]

Codecov Report

Merging #225 into master will increase coverage by 0.58%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
+ Coverage   51.74%   52.33%   +0.58%     
==========================================
  Files         125      125              
  Lines        5962     5962              
==========================================
+ Hits         3085     3120      +35     
+ Misses       2719     2665      -54     
- Partials      158      177      +19

Impacted Files	Coverage Δ
terraform/graylog/data_source_stream.go	55.44% <0%> (+2.97%)	⬆️
terraform/graylog/resource_stream.go	78.64% <0%> (+31.06%)	⬆️

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