refactor(sessionresolver): minor changes in files and types naming (#810)

Part of https://github.com/ooni/probe/issues/2135

internal/engine/internal/sessionresolver/jsoncodec_test.go

@@ -0,0 +1,48 @@
+package sessionresolver
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+type jsonCodecMockable struct {
+	EncodeData []byte
+	EncodeErr  error
+	DecodeErr  error
+}
+
+func (c *jsonCodecMockable) Encode(v interface{}) ([]byte, error) {
+	return c.EncodeData, c.EncodeErr
+}
+
+func (c *jsonCodecMockable) Decode(b []byte, v interface{}) error {
+	return c.DecodeErr
+}
+
+func TestJSONCodecCustom(t *testing.T) {
+	c := &jsonCodecMockable{}
+	reso := &Resolver{jsonCodec: c}
+	if r := reso.codec(); r != c {
+		t.Fatal("not the codec we expected")
+	}
+}
+
+func TestJSONCodecDefault(t *testing.T) {
+	reso := &Resolver{}
+	in := resolverinfo{
+		URL:   "https://dns.google/dns.query",
+		Score: 0.99,
+	}
+	data, err := reso.codec().Encode(in)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var out resolverinfo
+	if err := reso.codec().Decode(data, &out); err != nil {
+		t.Fatal(err)
+	}
+	if diff := cmp.Diff(in, out); diff != "" {
+		t.Fatal(diff)
+	}
+}