fix: exclude custom_logger from transport test-suite

[supervacuus]

A race in one of our unit tests was uncovered, probably due to recent fixes and updates in our ASAN runner config (#965), which may have influenced the timing.

The custom_logger test was written for a single-threaded configuration, but since it was automatically added to the transport test suite, the logger in this test would also be called from multiple threads, now leading to unexpected sequences in its assertions:

• https://github.com/getsentry/sentry-native/actions/runs/8388482978/job/22972769285#step:12:5003
• https://github.com/getsentry/sentry-native/actions/runs/8379423984/job/22947434351#step:12:4997
• https://github.com/getsentry/sentry-native/actions/runs/8379423984/job/22946299495#step:12:5038

The fix for this is a trivial exclusion from the transport test suite, but the fact that the race existed for several years in the tests is an excellent reason to add a note to the docs stating that synchronization is required in custom loggers 😅.

#skip-changelog

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.76%. Comparing base (f4b9cfa) to head (e9fe055).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #968      +/-   ##
==========================================
- Coverage   83.70%   82.76%   -0.95%     
==========================================
  Files          51       53       +2     
  Lines        6961     7444     +483     
  Branches     1093     1198     +105     
==========================================
+ Hits         5827     6161     +334     
- Misses       1030     1173     +143     
- Partials      104      110       +6