[TEST] Fix sync problems in OTLP File exporter tests.

[owent]

Fixes #3025

Changes

Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[marcalff]

LGTM, thanks for the test cleanup.

[marcalff]

The code change looks ok, but the test still fails under valgrind.

There must be another root cause to fix.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.60%. Comparing base (497eaf4) to head (6405b50).
Report is 117 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3031      +/-   ##
==========================================
+ Coverage   87.12%   87.60%   +0.49%     
==========================================
  Files         200      190      -10     
  Lines        6109     5870     -239     
==========================================
- Hits         5322     5142     -180     
+ Misses        787      728      -59     

see 123 files with indirect coverage changes

Need a second look, test still failing.

[owent]

nlohmann::json::parse seems parse json text failed, still try to find out the reason.

[marcalff]

nlohmann::json::parse seems parse json text failed, still try to find out the reason.

I have not found anything either.

Let's merge this, in hope that the next failure will bring more information about the root cause.

[marcalff]

Ok to merge.

This will help narrow the root cause, so it can be better investigated.