test/image: improve logging of failed osbuild execution

Previously, the test merged stderr and stderr and treated it as json. This was
obviously wrong. osbuild's stderr only returns python exception that are
of course not json.

The behaviour is now different:

1) stderr and stdout are treated separately
2) json from stdout is indented to prevent long lines. If it fails the test
   just prints the stdout as returned from osbuild.
3) stderr is printed as returned from osbuild

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

cmd/osbuild-image-tests/main_test.go

@@ -54,16 +54,25 @@ func runOsbuild(manifest []byte, store, outputDirectory string) error {
 	cmd := constants.GetOsbuildCommand(store, outputDirectory)
 
 	cmd.Stdin = bytes.NewReader(manifest)
-	var outBuffer bytes.Buffer
+	var outBuffer, errBuffer bytes.Buffer
 	cmd.Stdout = &outBuffer
-	cmd.Stderr = &outBuffer
+	cmd.Stderr = &errBuffer
 
 	err := cmd.Run()
 	if err != nil {
-		// Pretty print the osbuild error output.
-		buf := new(bytes.Buffer)
-		_ = json.Indent(buf, outBuffer.Bytes(), "", "    ")
-		fmt.Println(buf)
+		fmt.Println("stdout:")
+		// stdout is json, indent it, otherwise we get a huge one-liner
+		var formattedStdout bytes.Buffer
+		indentErr := json.Indent(&formattedStdout, outBuffer.Bytes(), "", "  ")
+		if indentErr == nil {
+			fmt.Println(formattedStdout.String())
+		} else {
+			// fallback to raw output if json indent failed
+			fmt.Println(outBuffer.String())
+		}
+
+		// stderr isn't structured, print it as is
+		fmt.Printf("stderr:\n%s", errBuffer.String())
 
 		return fmt.Errorf("running osbuild failed: %v", err)
 	}