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/main_test.go

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