update helm render tests to consider child charts values.yaml (#4725)

* update helm render tests to read child charts values.yaml

Helm installation by default, considers values.yaml for dependend charts
and uses them in rendering. This function is being used for add-ons to
keep the default template values, allowing further overriden from the
parent chart's i.e linkerd2 values.yaml or --addon-config through CLI.

This PR updates the Helm tests to reflect the same i.e consider
values.yaml of chart dependencies if present.

This does not have any UX changes but helps with the follow up 
add-on related work.

cli/cmd/install_helm_test.go

@@ -6,6 +6,7 @@ import (
 	"path/filepath"
 	"testing"
 
+	"github.com/linkerd/linkerd2/pkg/charts"
 	l5dcharts "github.com/linkerd/linkerd2/pkg/charts/linkerd2"
 	"github.com/linkerd/linkerd2/pkg/k8s"
 	"k8s.io/helm/pkg/chartutil"
@@ -230,6 +231,7 @@ func chartControlPlane(t *testing.T, ha bool, addOnConfig string, ignoreOutbound
 }
 
 func buildAddOnChart(t *testing.T, addon l5dcharts.AddOn, chartPartials *pb.Chart) *pb.Chart {
+	rawValues := readValuesFile(t, filepath.Join("add-ons", addon.Name()))
 	addOnChart := pb.Chart{
 		Metadata: &pb.Metadata{
 			Name: addon.Name(),
@@ -240,6 +242,9 @@ func buildAddOnChart(t *testing.T, addon l5dcharts.AddOn, chartPartials *pb.Char
 		Dependencies: []*pb.Chart{
 			chartPartials,
 		},
+		Values: &pb.Config{
+			Raw: string(rawValues),
+		},
 	}
 
 	for _, filepath := range append(addon.ConfigStageTemplates(), addon.ControlPlaneStageTemplates()...) {
@@ -291,3 +296,17 @@ func readTestValues(t *testing.T, ha bool, ignoreOutboundPorts string, ignoreInb
 
 	return yaml.Marshal(values)
 }
+
+// readValues reads values.yaml file from the given path
+func readValuesFile(t *testing.T, path string) []byte {
+
+	valuesFiles := []*chartutil.BufferedFile{
+		{Name: chartutil.ValuesfileName},
+	}
+
+	if err := charts.FilesReader(path+"/", valuesFiles); err != nil {
+		t.Fatal("Unexpected error", err)
+	}
+
+	return valuesFiles[0].Data
+}