Maint: do not mock os functions

- Use filepaths with /tmp for tests instead
- Only mock functions where filepath can't be specified such as user.Lookup

internal/provider/privateinternetaccess/provider.go

@@ -4,6 +4,7 @@ import (
 	"math/rand"
 	"time"
 
+	"github.com/qdm12/gluetun/internal/constants"
 	"github.com/qdm12/gluetun/internal/models"
 )
 
@@ -12,12 +13,18 @@ type PIA struct {
 	randSource   rand.Source
 	timeNow      func() time.Time
 	activeServer models.PIAServer
+	// Port forwarding
+	portForwardPath string
+	authFilePath    string
 }
 
-func New(servers []models.PIAServer, randSource rand.Source, timeNow func() time.Time) *PIA {
+func New(servers []models.PIAServer, randSource rand.Source,
+	timeNow func() time.Time) *PIA {
 	return &PIA{
-		servers:    servers,
-		timeNow:    timeNow,
-		randSource: randSource,
+		servers:         servers,
+		timeNow:         timeNow,
+		randSource:      randSource,
+		portForwardPath: constants.PIAPortForward,
+		authFilePath:    constants.OpenVPNAuthConf,
 	}
 }