Log debug output in helm.Config [#9]

Redacting KubeToken may not be sufficient, since it's possible that
someone would put secrets in Values or StringValues. Unilaterally
redacting those seems unhelpful, though, since they may be the very
thing the user is trying to debug. I've settled on redacting the obvious
field without trying to promise that all sensitive data will be hidden.

internal/helm/plan_test.go

@@ -4,7 +4,7 @@ import (
 	"fmt"
 	"github.com/golang/mock/gomock"
 	"github.com/stretchr/testify/suite"
-	"os"
+	"strings"
 	"testing"
 
 	"github.com/pelotech/drone-helm3/internal/run"
@@ -29,6 +29,8 @@ func (suite *PlanTestSuite) TestNewPlan() {
 	}
 	defer func() { help = origHelp }()
 
+	stdout := strings.Builder{}
+	stderr := strings.Builder{}
 	cfg := Config{
 		Command:      "help",
 		Debug:        false,
@@ -36,6 +38,8 @@ func (suite *PlanTestSuite) TestNewPlan() {
 		StringValues: "tensile_strength,flexibility",
 		ValuesFiles:  []string{"/root/price_inventory.yml"},
 		Namespace:    "outer",
+		Stdout:       &stdout,
+		Stderr:       &stderr,
 	}
 
 	runCfg := run.Config{
@@ -44,8 +48,8 @@ func (suite *PlanTestSuite) TestNewPlan() {
 		StringValues: "tensile_strength,flexibility",
 		ValuesFiles:  []string{"/root/price_inventory.yml"},
 		Namespace:    "outer",
-		Stdout:       os.Stdout,
-		Stderr:       os.Stderr,
+		Stdout:       &stdout,
+		Stderr:       &stderr,
 	}
 
 	stepOne.EXPECT().