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

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

internal/engine/internal/sessionresolver/codec_test.go

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