Autotest: enable colored test results.

Autotest: enable colored test results.

* lib/autotest/general.m4 (HELP_TUNING_BEGIN): New diversion.
(HELP_TUNING, HELP_OTHER, HELP_END): Bump diversion numbers.
(AT_INIT): Accept
--color and --color=never|auto|always.  If desired, colorize
test results and testsuite summary on standard output.
[HELP_TUNING]: Divert content instead to ...
[HELP_TUNING_BEGIN]: ... this diversion, m4_wrapped until the
end, when we know whether AT_COLOR_TESTS has been specified.
(AT_COLOR_TESTS): New macro, set the default for color to auto.
* doc/autoconf.texi (Writing Testsuites): Document it.
(testsuite Invocation): Document --color* options.
* tests/local.at: Call AT_COLOR_TESTS for Autoconf's testsuite.
* tests/autotest.at (color test results): New test, mirroring
color.test from Automake.
* NEWS: Update.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>