Example of how to test a lua plugin

[ckipp01]

Thanks a ton for this! It was one of the first things I came across when wanting to start nvim-metals, and this has been super helpful.

Another really helpful thing to include (which I'm currently digging through other plugins to find) is how to actually test a plugin. For example, using busted normally requires a src directory instead of the lua directory. To someone new to Lua this isn't the easiest thing to figure out. Also, vim isn't defined when testing as a standalone plugin, so how do you get around that?

I think this would be a great addition. :)