Implement colorized test output.
* automake.in (handle_tests): Set COLOR depending on `color-tests'.
* configure.ac (AM_INIT_AUTOMAKE): Use `color-tests'.
* doc/automake.texi (Tests, Options): Document test results,
document new option, document AM_COLOR_TESTS.
* lib/Automake/Options.pm (_process_option_list): Handle it.
* lib/am/check.am (am__tty_colors): New variable. Enable colored
output if TERM is not dumb and depending on AM_COLOR_TESTS and
whether stdout is connected to a terminal.
(check-TESTS): Use am__tty_colors.
* NEWS: Mention color-tests.
* tests/color.test: New test.
* tests/Makefile.am: Adjust.