fix(session): make sure tunnel code is tested (#301)

* fix(session): make sure tunnel code is tested

Part of https://github.com/ooni/probe/issues/985

* fix: add missing TunnelDir for correctness

internal/engine/session_nopsiphon_test.go

@@ -0,0 +1,20 @@
+// +build !ooni_psiphon_config
+
+package engine
+
+import (
+	"context"
+	"errors"
+	"testing"
+)
+
+func TestEarlySessionNoPsiphonFetchPsiphonConfig(t *testing.T) {
+	s := &sessionTunnelEarlySession{}
+	out, err := s.FetchPsiphonConfig(context.Background())
+	if !errors.Is(err, errPsiphonNoEmbeddedConfig) {
+		t.Fatal("not the error we expected", err)
+	}
+	if out != nil {
+		t.Fatal("expected nil here")
+	}
+}