fix: no tests found bug in python files

[olisikh]

It appears that the get_root function was unable to find the root folder, so tests in Python scripts were not executed, because neotest did not even try to discover them.

To fix that, I defined a get_root local function and reused it across the adapter.
This fixed an issue for me and I assume it should fix issue #83.

With this change, neotest-python discovers tests even if there are no special python files specified in a root folder.

[rcarriga]

This is very purposefully the case, otherwise when neotest is starting up and neotest-python says the current directory is the root, it will start scanning every file in the current directory which would not be desirable if opened in say the home directory.

If you really want to do this instead of just creating the files it looks for, you can do something like this

local custom_adapter = require("neotest-python")({})
local base_root = custom_adapter.root
custom_adapter.root = function(path)
  return base_root(path) or vim.loop.cwd() or ""
end