test: make the DDI tests work with older openssl

test: make the DDI tests work with older openssl

Older openssl unfortunately insists on having a config file with certain
fields, so let's reuse the one we already create in previous tests.

Should address following error on C8S:

[  407.812039] testsuite-50.sh[654]: + openssl req -config /dev/null -subj=/CN=waldo -x509 -sha256 -nodes -days 365 -newkey rsa:4096 -keyout /tmp/test-50-privkey.key -out /tmp/test-50-cert.crt
[  407.849089] testsuite-50.sh[2325]: Generating a RSA private key
[  408.947853] testsuite-50.sh[2325]: ..................................++++
[  423.100903] testsuite-50.sh[2325]: ..........++++
[  423.111036] testsuite-50.sh[2325]: writing new private key to '/tmp/test-50-privkey.key'
[  423.115036] testsuite-50.sh[2325]: -----
[  423.117842] testsuite-50.sh[2325]: unable to find 'distinguished_name' in config
[  423.120863] testsuite-50.sh[2325]: problems making Certificate Request
[  423.123448] testsuite-50.sh[2325]: 140737354091984:error:0E06D06C:configuration file routines:NCONF_get_string:no value:crypto/conf/conf_lib.c:273:group=req name=distinguished_name

Follow-up to 99d9edf0bde.