tests: Tweak the isolation wrapper for style and robustness.
tests: Tweak the isolation wrapper for style and robustness.

Followup to c01533920245599e4fbc4a1825520737b2c1a6ce.

Suggested-by: Matthew Martin

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