bpo-34130: Fix test_signal.test_warn_on_full_buffer() (GH-8327)

bpo-34130: Fix test_signal.test_warn_on_full_buffer() (GH-8327)

On Windows, sometimes test_signal.test_warn_on_full_buffer() fails to
fill the socketpair buffer. In that case, the C signal handler
succeed to write into the socket, it doesn't log the expected send
error, and so the test fail.

On Windows, the test now uses a timeout of 50 ms to fill the
socketpair buffer to fix this race condition.

Other changes:

* Begin with large chunk size to fill the buffer to speed up the
  test.
* Add error messages to assertion errors to more easily identify
  which assertion failed.
* Don't set the read end of the socketpair as non-blocking.