tests: Fix openssl ocsp command and check for errors

tests: Fix openssl ocsp command and check for errors

Fix the openssl ocsp command line and check if it returns an error - so
that instead of having something unusable later we error out
immediately. Moving the -sha256 argument earlier fixes hash function use
for the OCSP request generation (the old version used SHA-1).

Signed-off-by: Johannes Berg <johannes.berg@intel.com>