I think as all of these tests are buffer tests and not network tests, having them all use socket pair seems fine to me so let's just do that?

Originally posted by @mattklein123 in #10822

The underlying issue is that Unix can only use an AF_UNIX socketpair, while the current os_sys_calls_impl socketpair on Windows implements only AF_INET or AF_INET6.

In order to drop the pipe() in favor of a common use of socketpair() on both windows and linux in the test/common/buffer/watermark_buffer_test.cc and test/common/buffer/owned_impl_test.cc sources, we will need to implement AF_UNIX socketpair in source/common/api/win32/os_sys_calls_impl.cc - and must do so without using anonymous AF_UNIX sockets, which are unsupported on Windows (as on MacOS.)