Fix straightforward memory leaks in tests
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socket-service test: Don't leak writev vectors
Detected by AddressSanitizer.
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threadpool test: Don't leak the thread pool
Detected by AddressSanitizer.
socket-service test: Don't leak writev vectors
Detected by AddressSanitizer.
threadpool test: Don't leak the thread pool
Detected by AddressSanitizer.