REGTESTS: don't rely on the base64 utility when openssl base64 is already used

REGTESTS: don't rely on the base64 utility when openssl base64 is already used

Regtest ocsp_auto_update.vtc used to fail here on FreeBSD because the
base64 utility was not installed by default. Once installed it would
still fail because the utility doesn't support -w to wrap lines. Since
the regtest already relies on openssl base64 for a few commands, let's
just rely on it for the other ones. The only limitation is that openssl
freezes on lines longer than 1024 bytes, and doesn't seem to process more
than 255 chars at once, which might be the reason for using base64 -w 1000
in the first place (the script was probably tested like this). Instead
sed is efficient at wrapping long lines and does the job pretty well.
The output was fixed at 72 chars so that the output is also readable on
a terminal for debugging.