Changed the default get_test_iterations_without_progress_threshold() from 200

to 500.  Now that we have a better history management of loss values in the
trainer it's much more sensible to have a larger value here.

dlib/dnn/trainer.h

@@ -861,7 +861,7 @@ namespace dlib
             min_learning_rate = 1e-5;
             iter_without_progress_thresh = 2000;
             steps_without_progress = 0;
-            test_iter_without_progress_thresh = 200;
+            test_iter_without_progress_thresh = 500;
             test_steps_without_progress = 0;
 
             learning_rate_shrink = 0.1;

dlib/dnn/trainer_abstract.h

@@ -76,7 +76,7 @@ namespace dlib
                 - #get_learning_rate() == 1e-2 
                 - #get_min_learning_rate() == 1e-5
                 - #get_iterations_without_progress_threshold() == 2000
-                - #get_test_iterations_without_progress_threshold() == 200
+                - #get_test_iterations_without_progress_threshold() == 500
                 - #get_learning_rate_shrink_factor() == 0.1
                 - #get_learning_rate_schedule().size() == 0
                 - #get_train_one_step_calls() == 0