refactor(miniooni): divide et impera (#912)

This diff splits miniooni's implementation in smaller and more
easily tractable blocks ahead of future refactoring.

I'm trying to make `miniooni oonirun -i URL` as possible as
`miniooni -i URL oonirun`, because users typically expect this
kind of flexibity from modern Unix commands.

Part of https://github.com/ooni/probe/issues/2184

internal/cmd/miniooni/libminiooni_test.go

@@ -1,12 +0,0 @@
-package main
-
-import "testing"
-
-func TestSimple(t *testing.T) {
-	if testing.Short() {
-		t.Skip("skip test in short mode")
-	}
-	MainWithConfiguration("example", Options{
-		Yes: true,
-	})
-}