tap: some preparatory refactoring (1)

tap: some preparatory refactoring (1)

This refactoring is aimed at simplifying the introduction of
colored console output for the TAP driver.

* lib/tap-driver (console_output): Now accept two arguments, the
first one indicating which kind of thing is to be displayed (for
now only a test result or a diagnostic comment), and the second
one (if present) the message associated to it.
(handle_tap_test, handle_tap_comment, handle_tap_plan,
testsuite_error): Adapt to the new `console_output' interface.