vsock/test: add timeout_usleep() to allow sleeping in timeout sections

The timeout API uses signals, so we have documented not to use sleep(),
but we can use nanosleep(2) since POSIX.1 explicitly specifies that it
does not interact with signals.

Let's provide timeout_usleep() for that.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20250514141927.159456-2-sgarzare@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>