tests: move obsreport tests fixture to file

[paivagustavo]

Description: move tests fixture to file.

Link to tracking Issue: https://github.com/open-telemetry/opentelemetry-collector/pull/6346/files#r1010963714

[codecov]

Codecov Report

Base: 91.54% // Head: 91.57% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (7ec186d) compared to base (4e29c76).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6473      +/-   ##
==========================================
+ Coverage   91.54%   91.57%   +0.02%     
==========================================
  Files         238      238              
  Lines       13737    13737              
==========================================
+ Hits        12576    12580       +4     
+ Misses        930      927       -3     
+ Partials      231      230       -1     

Impacted Files	Coverage Δ
service/collector.go	78.26% <0.00%> (ø)
extension/ballastextension/memory_ballast.go	82.14% <0.00%> (ø)
processor/memorylimiterprocessor/memorylimiter.go	91.00% <0.00%> (+2.11%)	⬆️

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