Currently, the value of $COLORTERM in :terminal in tests depends on
outer environment because of 'notermguicolors'.
If $COLORTERM is not set in :terminal, an inner Nvim instance will try
to detect 'termguicolors' support, which may interfere with tests.
So set 'termguicolors' in outer Nvim instance unless $COLORTERM needs to
be overridden, and unset it in inner Nvim instance when running TUI.