test failure would not be reflected by the exit code of 'make'.
Setting a shell parameter in the left-hand side of a pipe is not visible to
commands after the pipe, because the left-hand side forks. (That's true both
in 'sh' used by 'make' and in 'zsh' that runs tests/test-highlighting.zsh, at
least on my system.) Therefore, move the colorizing hook to where it doesn't
interfere with setting the $something_failed (in tests/test-highlighting.zsh)
and $result (in Makefile) parameters.
