fix forcing test godebug on GoDebug without params

[gcollura]

The only way to trigger the .vscode/launch.json dropdown list was to be on a func main function, which is unlikely when you're trying to debug from a large Go project. go.nvim was assuming that GoDebug with no params, would always be on a Test* function, which was then causing the elif cfg_exist to never reach: https://github.com/gcollura/go.nvim/blob/18f782221a6a72840e9dcf4f5b13cbd5f2967c1d/lua/go/dap.lua?plain=1#L557

This PR checks a little more strictly that the user doesn't want to trigger a debug run on a "test", and it doesn't try to override the option passed by the user. For example it would set optarg['t'] = true even when another flag was passed.

[ray-x]

Thanks!