This creates a YAML test "features" that indices if the cluster being
tested has xpack installed (`xpack`) or if it does *not* have xpack
installed (`no_xpack`). It uses those features to centralize skipping
a few tests that fail if xpack is installed.
The plan is to use this in a followup to skip docs tests that require
xpack when xpack is not installed. We *plan* to use the declaration
of required license level on the docs page to generate the required
`skip`.
Closes#30933.
232c71b6bf