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/purevpn/portforward.go

@@ -7,11 +7,10 @@ import (
 
 	"github.com/qdm12/gluetun/internal/firewall"
 	"github.com/qdm12/golibs/logging"
-	"github.com/qdm12/golibs/os"
 )
 
 func (p *Purevpn) PortForward(ctx context.Context, client *http.Client,
-	openFile os.OpenFileFunc, pfLogger logging.Logger, gateway net.IP,
-	fw firewall.Configurator, syncState func(port uint16) (pfFilepath string)) {
+	pfLogger logging.Logger, gateway net.IP, fw firewall.Configurator,
+	syncState func(port uint16) (pfFilepath string)) {
 	panic("port forwarding is not supported for PureVPN")
 }