[Feature] Add variable indicator when running tests

[sharpchen]

Adding something like vim.g.plenary_busted_test to indicate the nvim instance is running busted test would be helpful for managing init.lua conditionally, something not related to testing might error and print extra message on test result window which I don't want.

For example, I have a vim.cmd.colo 'xamabah' in my init.lua, it always errors