Skip BuildIncremental_SimpleMvc_PersistsTargetInputFile on xplat

For some inexplicable reason timestamps returned as part of the test often show up
as being different resulting in test flakiness. We've manually verified that this
is not a product issue and builds are correctly incremental on xplat. See #2219
for past discussions.

Fixes https://github.com/aspnet/Razor/issues/2503

test/Microsoft.AspNetCore.Razor.Design.Test/IntegrationTests/BuildIncrementalismTest.cs

@@ -7,6 +7,7 @@ using System.IO;
 using System.Linq;
 using System.Runtime.InteropServices;
 using System.Threading.Tasks;
+using Microsoft.AspNetCore.Testing.xunit;
 using Xunit;
 
 namespace Microsoft.AspNetCore.Razor.Design.IntegrationTests
@@ -18,7 +19,8 @@ namespace Microsoft.AspNetCore.Razor.Design.IntegrationTests
         {
         }
 
-        [Fact]
+        [ConditionalFact]
+        [OSSkipCondition(OperatingSystems.MacOSX | OperatingSystems.Linux, SkipReason = "See https://github.com/aspnet/Razor/issues/2219")]
         [InitializeTestProject("SimpleMvc")]
         public async Task BuildIncremental_SimpleMvc_PersistsTargetInputFile()
         {
@@ -29,20 +31,13 @@ namespace Microsoft.AspNetCore.Razor.Design.IntegrationTests
             var result = await DotnetMSBuild("Build");
 
             var directoryPath = Path.Combine(result.Project.DirectoryPath, IntermediateOutputPath);
-            var filesToIgnore = new List<string>()
+            var filesToIgnore = new[]
             {
                 // These files are generated on every build.
                 Path.Combine(directoryPath, "SimpleMvc.csproj.CopyComplete"),
                 Path.Combine(directoryPath, "SimpleMvc.csproj.FileListAbsolute.txt"),
             };
 
-            if (RuntimeInformation.IsOSPlatform(OSPlatform.OSX) || RuntimeInformation.IsOSPlatform(OSPlatform.Linux))
-            {
-                // There is some quirkiness with MsBuild in unix where it regenerates this file
-                // even though it shouldn't. This is tracked here https://github.com/aspnet/Razor/issues/2219.
-                filesToIgnore.Add(Path.Combine(directoryPath, "SimpleMvc.TagHelpers.input.cache"));
-            }
-
             var files = Directory.GetFiles(directoryPath).Where(p => !filesToIgnore.Contains(p));
             foreach (var file in files)
             {