tests: Cover 'path_prefix'.
tests: Cover 'path_prefix'.

This is part of issue zsh-users/zsh-syntax-highlighting#198.

phy1729/zsh-syntax-highlighting,joshuarubin/dotfiles,zsh-users/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting,m0vie/zsh-syntax-highlighting,joshuarubin/dotfiles,zsh-users/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting-debian,joshuarubin/dotfiles,phy1729/zsh-syntax-highlighting,stuha/wbr-zsh,x-way/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting-debian,psprint/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,joshuarubin/dotfiles,x-way/zsh-syntax-highlighting,m0vie/zsh-syntax-highlighting,psprint/zsh-syntax-highlighting