Fixes #692.

This makes the tests more reproducable. In particular it avoids hiding
a WARN_CREATE_GLOBAL error when the dev happens to have defined that
variable in the environment (cf. next commit).

Useful when using, e.g., Debian's package, which outputs just "Debian" here.

Context: zsh-users/zsh-docker#2

Fixes zsh-users/zsh-syntax-highlighting#262.

Currently, 'make quiet-test' uses Perl.  However, since it is considered a development
tool rather than a user-facing tool, users and downstream packages needn't install Perl.
Furthermore, even this dev-only dependency may be dropped in the future.

The only difference between tests/tap-filter here and the one in the issue is using
a `cat` subshell v. using 'undef $/; <STDIN>'.

Change quoting to only quote the shell variable, not the make variable.  This
allows the leading '~' to be unescaped for make's sh to expand.

Followup to b1619c0013.

Patch-by: Matthew Martin

- Install the top-level README.md, which wasn't installed before.
- Install docs to /usr/local/share/doc rather than /usr/local/share.

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.

Color is only added when "make test"'s stdout is a tty.

Tested with 'bmake -B'.

Found-by: Carsten Hey

Don't modify $(PREFIX) from 'make', only from 'make install'.