Fix net452 build

src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/AnalyzerTest`1.cs

@@ -1273,11 +1273,13 @@ namespace Microsoft.CodeAnalysis.Testing
             static (string fileName, IEnumerable<string> folders) GetNameAndFoldersFromPath(string path)
             {
                 string fileName = Path.GetFileName(path);
-                string[] folders = Path.GetDirectoryName(path)!.Split(Path.DirectorySeparatorChar, StringSplitOptions.RemoveEmptyEntries);
+                string[] folders = Path.GetDirectoryName(path)!.Split(PathSplitChars, StringSplitOptions.RemoveEmptyEntries);
                 return (fileName, folders);
             }
         }
 
+        private static readonly char[] PathSplitChars = new[] { Path.DirectorySeparatorChar };
+
         /// <summary>
         /// Creates a solution that will be used as parent for the sources that need to be checked.
         /// </summary>