Add information-gathering about DNS resolvers configured to t_client.sh(.in)

Add information-gathering about DNS resolvers configured to t_client.sh(.in)

With the patchsets to add DNS configuration on Unix+MacOS systems, and
the addition of test stanzas to excercise and verify the OS specific
"dns-updown" script, it becomes important to trace test failures
("did it not ping because the DNS was not installed, or did something
else fail?") and also verify that DNS config is properly restored at
the end of each test.

Linux is probed with "resolvectl status", if available, and
"cat resolv.conf" if not.  MacOS uses scutil --dns.

All other platforms use "cat resolv.conf" for now (because even if
"a tool to maintain DNS config" is available, in the end resolv.conf
is always where the final config lands).

Include a bit of restructuring to handle linux iproute2 testing in the
"Linux" branch, and make the control flow more amenable to having a
second case / esac block.

Change-Id: I9cae7314203424e4a604073c5445559260172477
Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20250505142224.24935-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg31568.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>