Use 2048-bit keys in tests to enable tests on modern systems
    
    The crypto policies on some modern distributions don't allow 1024-bit
    keys. Without this change the LDAP tests won't work on these systems;
    OpenLDAP wouldn't start at all with messages like:
        slapd[32441]: main: TLS init def ctx failed: -1
        slapd[32441]: @(#) $OpenLDAP: slapd 2.4.46 (May 11 2018 17:12:39) $