vsock/test: Introduce vsock_bind_try() helper

vsock/test: Introduce vsock_bind_try() helper

Create a socket and bind() it. If binding failed, gracefully return an
error code while preserving `errno`.

Base vsock_bind() on top of it.

Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Reviewed-by: Luigi Leonardi <leonardi@redhat.com>
Link: https://patch.msgid.link/20250611-vsock-test-inc-cov-v3-1-5834060d9c20@rbox.co
Signed-off-by: Jakub Kicinski <kuba@kernel.org>