test(general): parse stdout and stderr in robustness framework helper (#2779)

Parse stdout as well as stderr in `KopiaSnapshotter.CreateSnapshot`.
This is contained to the Robustness Framework.

Co-authored-by: Carl Braganza <carl@kasten.io>

tests/tools/kopiarunner/kopia_snapshotter.go

@@ -132,11 +132,15 @@ func (ks *KopiaSnapshotter) ConnectOrCreateFilesystem(repoPath string) error {
 // CreateSnapshot implements the Snapshotter interface, issues a kopia snapshot
 // create command on the provided source path.
 func (ks *KopiaSnapshotter) CreateSnapshot(source string) (snapID string, err error) {
-	_, errOut, err := ks.Runner.Run("snapshot", "create", parallelFlag, strconv.Itoa(parallelSetting), noProgressFlag, source)
+	stdOut, errOut, err := ks.Runner.Run("snapshot", "create", parallelFlag, strconv.Itoa(parallelSetting), noProgressFlag, source)
 	if err != nil {
 		return "", err
 	}
 
+	if stdOut != "" {
+		return parseSnapID(strings.Split(stdOut, "\n"))
+	}
+
 	return parseSnapID(strings.Split(errOut, "\n"))
 }