introduce V= level to manage t_client.sh output verbosity

introduce V= level to manage t_client.sh output verbosity

If t_client.sh is run interactively, more verbose output is useful
to quickly see what it is doing.  If run from a CI environment, going
through lots of output for successful tests just to find the one that
failed is non-useful.

Introduce V=<n> environment variable to control output verbosity

 V=0 - do not print any per-test output at all, just overall summary
 V=1 - print single header line for each successful test
       print full output for failing tests
 V=99 - print full output, always, as before

default is V=1 now

Signed-off-by: Gert Doering <gert@greenie.muc.de>
v2:
 fix erroneous test on "-n"
 do not accumulate extra "\n" in outbuf (V=1)
 fix missing "-e" at "test failures. FAIL." message
 fix missing "\n" when including "diff" output
 fix missing "-n" when printing outbuf (= extra newline)
 (and more newlines being shuffled around)

v3:
 fix quoting on inclusion of "ifconfig/route diff", with newlines...

Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <20220920132351.27718-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25285.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>