test-explorer: auto update xgo if test.config.json says 'xgo.auto_update: true'

[xhd2015]

And possibly prompt in the test explorer UI.

Update check: start a background goroutine, run xgo tool check-update --json to get if there is newer version.

If the result says there is new update, then run go install github.com/xhd2015/xgo/cmd/xgo@latest

[xhd2015]

The test.config.json:

{
    "xgo":{
        "auto_update":true
    }
}

If this option is true, then upon every invoke to xgo e, go install github.com/xhd2015/xgo/cmd/xgo@latest will be executed in a standalone goroutine silently.

This behavior can be turned off by setting XGO_AUTO_UPDATE=never.