Nondeterministic test failure with AsyncStream + Dispatch interop

Occasionally tests such as `linkerSignedBinaryDetection` will fail with an error message like:

    Caught error: dataStream(reading: DispatchFD(rawValue: 9))#1(): Is a directory (21)

This is due to the file descriptor that is read from in Dispatch+Async.swift becoming invalid, but it's not clear why this happens.