Only error for duplicate output dirs on CI builds

Directory.Build.props

@@ -17,6 +17,8 @@
     <DisableImplicitPackageTargetFallback>true</DisableImplicitPackageTargetFallback>
 
     <!-- <ArtifactsShippingSymbolsDir>$(ArtifactsDir)symbols\$(Configuration)\Shipping</ArtifactsShippingSymbolsDir> -->
+
+    <DefineConstants Condition="'$(ContinuousIntegrationBuild)' == 'true'">$(DefineConstants);CI_BUILD</DefineConstants>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(IsTestProject)' == 'true'">

src/Tests/Microsoft.NET.TestFramework/TestAssetsManager.cs

@@ -140,11 +140,12 @@ namespace Microsoft.NET.TestFramework
             }
 
             var directoryPath = Path.Combine(baseDirectory, directoryName.ToString());
-
+#if CI_BUILD
             if (Directory.Exists(directoryPath))
             {
                 throw new Exception($"Test dir {directoryPath} already exists");
             }
+#endif
 
             return directoryPath;
         }