Fix global config for functional tests

[hjoliver]

• fix documentation and code associated with special global config file for the test battery (the installed global config - if any - has to be ignored by the tests)
  • restores cylc get-global --print-site-dir option used in the test header (we removed the option back in April for unknown reasons - accidentally?)
  • allows test and normal global config under /etc (site) and ~/.cylc (user)
  • corrects a few remaining isntances of the of old "global.rc" filename
• removes outdated conda recipe from cylc-flow package resources

@wxtim - this makes use of flow.rc and flow-tests.rc consistent throughout cylc-flow for current usage; I think it will change to cylc-flow.rc later your config unification work

@matthewrmshin - have you been using a site config file for Cylc 8 tests? Seems like it would be broken at the moment. Either way, maybe you can check this while we still have you on the job?

This is a small change with no associated Issue.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).

• Already covered by existing tests.

• No change log entry required (why? e.g. invisible to users).

• No documentation update required.

[matthewrmshin]

No, I have been running the test battery with no configuration since the Python 3 + ZMQ port - as we are still trying to figure out how to deploy Cylc 8 to some of our remote environments that have no Internet access. Now that we have the Conda environment, we should spend some time to get that sorted (noting cylc/cylc-admin#27 and #3376 ).

[wxtim]

• I've eyeballed it and am reasonably happy with the changes.
• I have not yet got it working on my local machine, probably becuase I'm picking up an unwanted .rc file

[wxtim]

Now confident of tests. Sorry about the delay. :(