Return valid json in Server fixture

[heeckhau]

• return json instead of string
• removed trailing whitespace
• use a constant for the default port
• give binary a better name

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 54.55%. Comparing base (6344410) to head (dd34c28).
Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
crates/server-fixture/server/src/lib.rs	91.83%	4 Missing ⚠️
crates/server-fixture/server/src/main.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #652      +/-   ##
==========================================
+ Coverage   54.35%   54.55%   +0.19%     
==========================================
  Files         192      192              
  Lines       20473    20518      +45     
==========================================
+ Hits        11129    11194      +65     
+ Misses       9344     9324      -20     

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[yuroitaki]

nice stuff!