vsock/test: retry send() to avoid occasional failure in sigpipe test

vsock/test: retry send() to avoid occasional failure in sigpipe test

When the other peer calls shutdown(SHUT_RD), there is a chance that
the send() call could occur before the message carrying the close
information arrives over the transport. In such cases, the send()
might still succeed. To avoid this race, let's retry the send() call
a few times, ensuring the test is more reliable.

Sleep a little before trying again to avoid flooding the other peer
and filling its receive buffer, causing false-negative.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20250514141927.159456-3-sgarzare@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>