Refactor logLines to SplitByNewline, deduplicate

internal/testutil/stringutil/stringutil.go

@@ -0,0 +1,14 @@
+// Copyright 2024 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package stringutil
+
+import "strings"
+
+func SplitByNewline(logs string) []string {
+	if len(logs) == 0 {
+		return nil
+	}
+
+	return strings.Split(strings.TrimSpace(logs), "\n")
+}