[NO TICKET] more debug logging, do not skip tests when running in forked processes

[anmarchenko]

What does this PR do?
A couple of smaller changes ahead of ITR public beta:

• more debug logging
• error handling for coverage writer: instead of silently killing the the coverage writer on errors print WARN messages and back-off
• do not skip tests when using forked test runners (to be supported later)
• disable startup logging from tracing as it is useless for test visibility mode and spams logs

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 46.15385% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 98.98%. Comparing base (4534437) to head (2bea9f7).

Files	Patch %	Lines
lib/datadog/ci/itr/coverage/writer.rb	0.00%	5 Missing ⚠️
lib/datadog/ci/itr/runner.rb	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #168      +/-   ##
==========================================
- Coverage   99.04%   98.98%   -0.07%     
==========================================
  Files         198      198              
  Lines        9627     9638      +11     
  Branches      421      423       +2     
==========================================
+ Hits         9535     9540       +5     
- Misses         92       98       +6     

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