fix(test_harness): followup to #459, dont use --clean but rather --noplugin

[Conni2461]

otherwise its a breaking change, see
nvim-treesitter/nvim-treesitter#4856

[wookayin]

A consequence of this change is that, by default a non-clean, non-plugin neovim will run: this means that (i) the default init.lua is executed but (ii) no plugins will be loaded. Isn't a weird combination? I feel a breaking change like #459 is needed and all the downstream projects should migrate to the behavior by explicitly specifying init files.

Therefore, if user's init.lua is using plenary.nvim in their normal config (which I think is quite usual), the startup process will crash because plenary won't be available due to suppressed plugin loading, until init.lua is complete and set rtp is executed.

Error detected while processing ~/.config/nvim/init.lua:
E5113: Error while calling lua chunk:
~/.config/nvim/lua/init.lua:XX: module 'plenary.path' 
not found:
        no field package.preload['plenary.path']

[Conni2461]

please open a new issue, then we can have a discussion on how we would solve this issue.