Add TAP tests for include directives in HBA end ident files

Add TAP tests for include directives in HBA end ident files

This commit adds a basic set of authentication tests to check after the
new keywords added by a54b658 for the HBA and ident files, aka
"include", "include_if_exists" and "include_dir".

This includes checks for all the positive cases originally proposed,
where valid contents are generated for the HBA and ident files without
any errors happening in the server, checking as well the contents of
their respective system views.  The error handling will be evaluated
separately (-DEXEC_BACKEND makes that trickier), and what we have here
covers most of the ground I would like to see covered if one manipulates
the tokenization logic of hba.c in the future.

While on it, some coverage is added for files included with '@' for
database or user name lists.

Author: Julien Rouhaud
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/20220223045959.35ipdsvbxcstrhya@jrouhaud