Increase fudge factor in FileSystemWatcher_Error_File to force the error event to occur.

I tried to be clever in the calculation, but really we don't need to be very accurate so long as we can force the error.  This is still not fool-proof as it seems a stressed system may drop/coalesce some of these events (perhaps because they are touching the same file & time) but it is close enough.

[tfs-changeset: 1493258]

Author: ericstj

src/System.IO.FileSystem.Watcher/tests/FileSystemWatcher.Error.cs

@@ -63,8 +63,8 @@ public static void FileSystemWatcher_Error_File()
             int internalBufferOperationCapacity = watcher.InternalBufferSize / (12 + 2 * (file.Path.Length + 1));
 
             // generate enough file change events to overflow the buffer
-            // 2x is an approximation that should force the overflow.
-            for (int i = 1; i < internalBufferOperationCapacity * 2; i++)
+            // 4x is an approximation that should force the overflow.
+            for (int i = 1; i < internalBufferOperationCapacity * 4; i++)
             {
                 File.SetLastWriteTime(file.Path, DateTime.Now + TimeSpan.FromSeconds(i));
             }