refactor(engine): *http.Client -> model.HTTPClient (#836)

This diff makes the implementation of the engine package more
abstract by changing HTTPClient() to return a model.HTTPClient
as opposed to returning an *http.Client.

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

internal/model/experiment.go

@@ -2,7 +2,6 @@ package model
 
 import (
 	"context"
-	"net/http"
 )
 
 //
@@ -13,7 +12,7 @@ import (
 // ExperimentSession is the experiment's view of a session.
 type ExperimentSession interface {
 	GetTestHelpersByName(name string) ([]OOAPIService, bool)
-	DefaultHTTPClient() *http.Client
+	DefaultHTTPClient() HTTPClient
 	FetchPsiphonConfig(ctx context.Context) ([]byte, error)
 	FetchTorTargets(ctx context.Context, cc string) (map[string]OOAPITorTarget, error)
 	FetchURLList(ctx context.Context, config OOAPIURLListConfig) ([]OOAPIURLInfo, error)