test/bioprinttest.c: move the %n result to the field that is later checked

test/bioprinttest.c: move the %n result to the field that is later checked

The test_n test implicitly assumed a certain union layout, as the %n was
writing to the union field in accordance with the length modifier being
tested, but comparison of the expected value was dan agains the val field,
and that is incorrect, especially on big-endian architectures.  Fix that
by explicitly assigning the result to the val field of the union and
updating the expected values where the resulting value overflows
into negative.

Fixes: 9deaf8383338 "test/bioprinttest.c: add some checks for integer and string printing"
Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28388)