fix(kumactl) use strings for stderr output test check (#2323)

If we assert that kumactl standard error is empty using a byte buffer,
then the Ginkgo error shows a hex dump, which isn't that easy to read.
Switch to checking for an empty standard error string so that Ginkgo
shows any error messages directly in the test failure.

Signed-off-by: James Peach <james.peach@konghq.com>

app/kumactl/cmd/completion/completion_test.go

@@ -40,7 +40,7 @@ var _ = Describe("kumactl completion", func() {
 
 			// then
 			Expect(err).ToNot(HaveOccurred())
-			Expect(stderr.Bytes()).To(BeNil())
+			Expect(stderr.String()).To(BeEmpty())
 
 			// and
 			actual := stdout.Bytes()

app/kumactl/cmd/install/install_control_plane_test.go

@@ -71,7 +71,7 @@ var _ = Describe("kumactl install control-plane", func() {
 
 			// then command succeed
 			Expect(err).ToNot(HaveOccurred())
-			Expect(stderr.Bytes()).To(BeNil())
+			Expect(stderr.String()).To(BeEmpty())
 
 			// and output matches golden files
 			actual := stdout.Bytes()

app/kumactl/cmd/install/install_crds_test.go

@@ -38,7 +38,7 @@ var _ = Describe("kumactl install crds", func() {
 			// then
 			Expect(err).ToNot(HaveOccurred())
 			// and
-			Expect(stderr.Bytes()).To(BeNil())
+			Expect(stderr.String()).To(BeEmpty())
 
 			// when
 			actual := stdout.Bytes()

app/kumactl/cmd/install/install_demo_test.go

@@ -50,7 +50,7 @@ var _ = Describe("kumactl install demo", func() {
 
 			// then
 			Expect(err).ToNot(HaveOccurred())
-			Expect(stderr.Bytes()).To(BeNil())
+			Expect(stderr.String()).To(BeEmpty())
 
 			// and output matches golden files
 			actual := stdout.Bytes()

app/kumactl/cmd/install/install_gateway_test.go

@@ -50,7 +50,7 @@ var _ = Describe("kumactl install gateway", func() {
 
 			// then
 			Expect(err).ToNot(HaveOccurred())
-			Expect(stderr.Bytes()).To(BeNil())
+			Expect(stderr.String()).To(BeEmpty())
 
 			// and output matches golden files
 			actual := stdout.Bytes()

app/kumactl/cmd/install/install_logging_test.go

@@ -50,7 +50,7 @@ var _ = Describe("kumactl install logging", func() {
 
 			// then
 			Expect(err).ToNot(HaveOccurred())
-			Expect(stderr.Bytes()).To(BeNil())
+			Expect(stderr.String()).To(BeEmpty())
 
 			// and output matches golden files
 			actual := stdout.Bytes()

app/kumactl/cmd/install/install_metrics_test.go

@@ -52,7 +52,7 @@ var _ = Describe("kumactl install metrics", func() {
 
 			// then
 			Expect(err).ToNot(HaveOccurred())
-			Expect(stderr.Bytes()).To(BeNil())
+			Expect(stderr.String()).To(BeEmpty())
 
 			// and output matches golden files
 			actual := stdout.Bytes()

app/kumactl/cmd/install/install_tracing_test.go

@@ -39,7 +39,7 @@ var _ = Describe("kumactl install tracing", func() {
 
 			// then
 			Expect(err).ToNot(HaveOccurred())
-			Expect(stderr.Bytes()).To(BeNil())
+			Expect(stderr.String()).To(BeEmpty())
 
 			// and output matches golden files
 			actual := stdout.Bytes()

app/kumactl/cmd/install/install_transparent_proxy_test.go

@@ -42,7 +42,7 @@ var _ = Describe("kumactl install tracing", func() {
 			// then
 			Expect(err).ToNot(HaveOccurred())
 			// and
-			Expect(stderr.Bytes()).To(BeNil())
+			Expect(stderr.String()).To(BeEmpty())
 
 			// when
 			regex, err := ioutil.ReadFile(filepath.Join("testdata", given.goldenFile))

app/kumactl/cmd/uninstall/uninstall_transparent_proxy_test.go

@@ -41,7 +41,7 @@ var _ = Describe("kumactl install tracing", func() {
 			// then
 			Expect(err).ToNot(HaveOccurred())
 			// and
-			Expect(stderr.Bytes()).To(BeNil())
+			Expect(stderr.String()).To(BeEmpty())
 
 			// when
 			regex, err := ioutil.ReadFile(filepath.Join("testdata", given.goldenFile))