tests: Add a 'print failures only' mode to 'make test', called 'make quiet-test'.

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>'.

tests/tap-filter

@@ -0,0 +1,45 @@
+#!/usr/bin/env perl
+# -------------------------------------------------------------------------------------------------
+# Copyright (c) 2015 zsh-syntax-highlighting contributors
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without modification, are permitted
+# provided that the following conditions are met:
+#
+#  * Redistributions of source code must retain the above copyright notice, this list of conditions
+#    and the following disclaimer.
+#  * Redistributions in binary form must reproduce the above copyright notice, this list of
+#    conditions and the following disclaimer in the documentation and/or other materials provided
+#    with the distribution.
+#  * Neither the name of the zsh-syntax-highlighting contributors nor the names of its contributors
+#    may be used to endorse or promote products derived from this software without specific prior
+#    written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+# IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+# FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER
+# IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+# -------------------------------------------------------------------------------------------------
+# vim: ft=perl sw=2 ts=2 et
+# -------------------------------------------------------------------------------------------------
+
+# This is a stdin-to-stdout filter that takes TAP output (such as 'make test')
+# on stdin and deletes lines pertaining to expected results.
+#
+# More specifically, if any of the test points in a test file either failed but
+# was expected to pass, or passed but was expected to fail, then emit that test
+# file's output; else, elide that test file's output.
+
+use v5.10.0;
+use warnings;
+use strict;
+
+undef $/; # slurp mode
+print for
+  grep { /^ok.*# TODO/m or /^not ok(?!.*# TODO)/m }
+    split /^(?=#)/m,
+      <STDIN>;