thread count discrepancy in extern-use test

[acarioni]

The extern-use test is currently failing due to an unexpected number of attached threads. It expects 4 threads, but the actual count is 5.

This issue was encountered after compiling the code with Haxe 4.3.3. I tested against both the latest hxcpp version available on haxelib (4.3.2) and the most recent tag on GitHub (4.3.23). The test environment is a Mac Mini M2 running Sonoma 14.4.1.