TAP syntax violation: "Bail out" stdout/stderr

[test/test-harness.zsh](https://github.com/zsh-users/zsh-syntax-highlighting/blob/f73f3d53d3a6cc27e63e7008f7437d0f41f5b484/tests/test-highlighting.zsh) emits `Bail out!` lines to stderr, although the TAP specification says they should be printed to stdout:

http://testanything.org/tap-specification.html#bail-out
