nixosTests.prosody: extend- self-signed cert expiration date

The test certificate expiration date was set to the default 30 days.
This certificate is generated through its own derivation. As with
every derivation, it gets cached by cache.nixos.org once we build it.

In practice, we rebuild this derivation only if one of its input
changes. The only inputs here being openssl and stdenv.

While it's not an issue on the unstable branches, it can be
problematic on a stable release: the test will fail after 30 days.

Extending the certificate lifespan from 1 month to 100 years to prevent
it from getting expired while being cached.

See
https://github.com/NixOS/nixpkgs/pull/132898#issuecomment-894495057
for more context.

nixos/tests/xmpp/prosody.nix

@@ -1,6 +1,6 @@
 let
   cert = pkgs: pkgs.runCommandNoCC "selfSignedCerts" { buildInputs = [ pkgs.openssl ]; } ''
-    openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -nodes -subj '/CN=example.com/CN=uploads.example.com/CN=conference.example.com'
+    openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -nodes -subj '/CN=example.com/CN=uploads.example.com/CN=conference.example.com' -days 36500
     mkdir -p $out
     cp key.pem cert.pem $out
   '';