fix(test): close TSAN race in WaitForSocketFile helper

[pypingou]

WaitForSocketFile() used stat() to detect when the server's socket file appeared, but that only proves bind() completed — not listen(). Under TSAN instrumentation the window between bind() and listen() widens enough for the subsequent connect() in the test to hit ECONNREFUSED.

Replace the stat()-based check with an actual connect() probe that retries until the server is accepting connections, closing the race.

The probe connection is immediately closed without sending data, so the server treats it as a closed-by-peer with no session — no factory calls or session counters are affected.

See: https://github.com/eclipse-score/logging/actions/runs/35837155339/job/107103240555 for a example where this race condition happened.

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