neovim: test that neovim's config loads properly

[Cedric Halbronn]

following this issue saidelike#2 (comment)

It would be nice if we could see that error message directly on ci using one of the following:

• we could run nvim twice, once to detect errors with the config being loaded and a second one to actually run the test
• or we could redirect the logs for the config into a file and just show that when we fail to read the logfile for running the test