Improved readability of tests

[WilliamBergamin]

Improved the readability of newly created test to:

• facilitate the understanding of http status codes in tests
• ensure that content types and other web standards are maintained

I don't know if we want to implement this for every test in the library, maybe someone can comment on this?

Category (place an x in each of the [ ])

• bolt (Bolt for Java)
• bolt-{sub modules} (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

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[seratch]

Thanks for the changes. Can you make the constants static?

[codecov]

Codecov Report

Merging #1025 (731976d) into main (6b8fc68) will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main    #1025   +/-   ##
=========================================
  Coverage     76.78%   76.79%           
- Complexity     3705     3706    +1     
=========================================
  Files           406      406           
  Lines         11159    11159           
  Branches       1112     1112           
=========================================
+ Hits           8568     8569    +1     
  Misses         1921     1921           
+ Partials        670      669    -1     

Impacted Files	Coverage Δ
...imits/metrics/impl/BaseMemoryMetricsDatastore.java	80.08% <0.00%> (+0.43%)	⬆️

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