Treat an inaccessible tray mutex as no tray

src/DiffEngine/Tray/DiffEngineTray.cs

@@ -17,6 +17,13 @@ static DiffEngineTray()
         catch (IOException)
         {
         }
+        // A mutex that exists but is not accessible to this caller, which Mutex.TryOpenExisting
+        // documents: the tray under one account and the tests under another. Uncaught in a static
+        // constructor it is far worse than a wrong answer - the type never initialises, so every
+        // later DiffRunner.Launch and AddDelete in the process throws TypeInitializationException
+        catch (UnauthorizedAccessException)
+        {
+        }
     }
 
     public static bool IsRunning { get; internal set; }

src/DiffEngine/Tray/TrayDetector.cs

@@ -15,6 +15,13 @@ public static bool IsRunning()
         catch (IOException)
         {
         }
+        // Documented for a mutex that exists but cannot be opened by this caller: the tray running
+        // under one account and the tests under another in the same session. "Not accessible" is
+        // not "not running", but from here there is nothing to tell them apart with, and the
+        // answer that keeps a diff tool launching is the one to take
+        catch (UnauthorizedAccessException)
+        {
+        }
 
         return false;
     }