Make MNIST dataset name more meaningful

Author: oliag

examples/datasets/Train-Tiny-28x28.txt renamed to examples/datasets/MNIST-Train-Tiny-28x28.txt

@@ -376,7 +376,7 @@ public void TestCrossValidationMacro()
         [Fact]
         public void TestCrossValidationMacroWithMultiClass()
         {
-            var dataPath = GetDataPath(@"Train-Tiny-28x28.txt");
+            var dataPath = GetDataPath(@"MNIST-Train-Tiny-28x28.txt");
             using (var env = new TlcEnvironment(42))
             {
                 var subGraph = env.CreateExperiment();

test/Microsoft.ML.Core.Tests/UnitTests/TestCSharpApi.cs

@@ -1918,7 +1918,7 @@ public void EntryPointTrainTestMacroNoTransformInput()
         [Fact]
         public void EntryPointKMeans()
         {
-            TestEntryPointRoutine("Train-Tiny-28x28.txt", "Trainers.KMeansPlusPlusClusterer");
+            TestEntryPointRoutine("MNIST-Train-Tiny-28x28.txt", "Trainers.KMeansPlusPlusClusterer");
         }
 
         [Fact]