Unify whitespace characters to fix tests

DiscordChatExporter.Cli.Tests/Utils/Extensions/StringExtensions.cs

@@ -0,0 +1,16 @@
+using System.Text;
+
+namespace DiscordChatExporter.Cli.Tests.Utils.Extensions;
+
+internal static class StringExtensions
+{
+    public static string ReplaceWhiteSpace(this string str, string replacement = " ")
+    {
+        var buffer = new StringBuilder(str.Length);
+
+        foreach (var ch in str)
+            buffer.Append(char.IsWhiteSpace(ch) ? replacement : ch);
+
+        return buffer.ToString();
+    }
+}