Package tests should test using a custom ES_PATH_CONF

[jasontedor]

Our RPM and Debian packages will now respect ES_PATH_CONF during installs, upgrades, and package removal. We do not have coverage for this though, our packaging tests use the default of /etc/elasticsearch. In order to ensure that we maintain this behavior, our package tests should also run in a mode where they run the entire suite with a custom ES_PATH_CONF (maybe randomized with a space in the name, to ensure that we have the quoting correct too).

Relates #50158
Relates #50246

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Packaging)

[rjernst]

I started trying to add this to the packaging tests, but stopped because this requires copying /etc/elasticsearch contents to the alternate ES_PATH_CONF after installing. It would also mean updating the elasticsearch package would not pickup updates to configuration files, even ones that may not be modified often by users like the log4j config. We may want to rethink this feature (and #24867) as this seems pretty difficult as it is for users to implement, so I'm not sure what the added benefit is.

[mmomtchev]

Just a quick reminder that the official Puppet modules will empty /etc/default/elasticsearch of all configuration, including ES_PATH_CONF, effectively rendering elasticsearch unable to upgrade through the deb package