tap: support colorization of testsuite progress output

tap: support colorization of testsuite progress output

* lib/tap-driver (%COLORS): New variable (definition extracted
from `lib/am/check.am:$(am__tty_colors)', with some obvious
adjustments.
(report): Adjust to colorize console output when required,
using ...
(decorate_result): ... this new function.
(colored): New function, used by the one above.
* tests/tap-summary.test: Also run the checks when `color-tests'
is in use.
* tests/Makefile.am (XFAIL_TESTS): Remove `tap-color.test'.