test_create_read_special_symlink broken - just hangs infinitely

[ThomasWaldmann]

master branch.

likely broken due to threading usage in test.

iirc one could reproduce with pytest -k test_create_read_special_symlink - it will never terminate.

[ThomasWaldmann]

btw: not fixed by #7317.

[ThomasWaldmann]

I now ran into similar hanging issues on Linux (Github CI runners) within PR #8883 - although it never was an issues on Linux (master branch).

Tried to fix by:

• adding a delay between thread start and borg create
• adding a synchronisation at the same place
• reading all remaining data from the pipe, using multiple reads until no data is returned anymore

All did not fix the hanging.