feat(miniooni): optionally log using emojis (#932)

As silly as it seems, emojis help _a lot_ when eyeballing logs
to quickly identify unexpected lines.

I'm doing this work as part of https://github.com/ooni/probe/issues/2257

internal/model/mocks/writer.go

@@ -0,0 +1,16 @@
+package mocks
+
+import "io"
+
+// Writer allows to mock any io.Writer.
+type Writer struct {
+	MockWrite func(b []byte) (int, error)
+}
+
+// Writer implements an io.Writer.
+var _ io.Writer = &Writer{}
+
+// Write implements io.Writer.Write.
+func (r *Writer) Write(b []byte) (int, error) {
+	return r.MockWrite(b)
+}