Regression tests that exercise encryption

[cormacrelf]

I would like dpxcrypt to be reduced essentially to a few short functions using RustCrypto crates. This can’t be done properly until there are test cases that are encrypted — I would be surprised if any arxiv papers were.

how xdvipdfmx encryption is enabled

Tectonic does not allow enabling encryption from the command line, so the encryption special needs to be inserted into the tex sources. Maybe this could be automated for a small portion of the arxiv library. What do you think @Mrmaxmeier?

[burrbull]

cc @pkgw

[pkgw]

With our new support for -Z unstable options, that could be a good mechanism for enabling encryption through the CLI.