selftest/sstatetests: run CDN check twice, ignoring errors the first time

selftest/sstatetests: run CDN check twice, ignoring errors the first time

The current CDN isn't able to serve all objects on first request, and it
was suggested to work around that by trying again:
https://bugzilla.yoctoproject.org/show_bug.cgi?id=15335#c16

Once CDN moves to a better location this can probably be reverted.

[YOCTO #15335]

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>