test/params_api_test.c: Correct the checks of OSSL_PARAM_set_BN()
test/params_api_test.c: Correct the checks of OSSL_PARAM_set_BN()

Now, the returned size check matches the check made for all other
integer types.

Reviewed-by: Patrick Steuer <0d1e6eb02b051a0fb05954a8a115445ceed2dd93@de.ibm.com>
(Merged from https://github.com/openssl/openssl/pull/10326)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl