The functions can remain defined because, nowadays, the test harness
runs each test in a subshell; but that's a well-known property of the
test harness so need not be mentioned explicitly.
Inspired by discussion on issue #443.
All these files should be sourced, not executed; and Debian's lintian complains:
W: zsh-syntax-highlighting: script-not-executable usr/share/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh
