* bpo-34130: Fix test_signal.test_socket() (GH-8326)

test_signal.test_socket(): On Windows, sometimes even if the C signal handler
succeed to write the signal number into the write end of the socketpair, the
test fails with a BlockingIOError on the non-blocking read.recv(1) because the
read end of the socketpair didn't receive the byte yet.

Fix the race condition on Windows by setting the read end as blocking.

(cherry picked from commit 99bb6df6)

* 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.

(cherry picked from commit 686b4b5f)