The new failure mode is:
.
    Running test main
    Bail out! On './highlighters/main/test-data/glob.zsh': Either 'PREBUFFER' or 'BUFFER' must be declared and non-blank
    Bail out! On './highlighters/main/test-data/glob.zsh': output on stderr
    Running test pattern

Fixes #478.

Remove superfluous single quotes around the second hyphen.

Closes #291.

Use 'print -r --' instead, so literal backslashes in $BUFFER aren't expanded.

Found-by: Matthew Martin

The new output improves readability of the 'main' highlighter's test output.

Since the _zsh_highlight_add_highlight that the tests use ignores
ZSH_HIGHLIGHT_STYLES, we can test both an injective mapping and an empty
mapping at once.

Closes #287. Testing the keys directly removes the need for
unused_highlight which will be removed next.

'none' is a valid highlighting type. Since by convention styles are all
lowercase, 'NONE' shouldn't conflict with anything.

Closes #289.

In basic regular expressions | is not a special character that spearates
branches, so use an extended regular expression.

No functional change, except that error messages are now more readable.

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>'.