Test Infrastructure Updates

Creates a TestDirectory abstraction under TestInstance to manage creation of test-specific working directories
Enables TestAssetManager to create TestDirectory instances
Enables fluent addition of Environment Variables to TestCommand
Adds PathUtility support for ensuring a directory exists

test/Microsoft.DotNet.Tools.Tests.Utilities/Commands/TestCommand.cs

@@ -193,5 +193,12 @@ namespace Microsoft.DotNet.Tools.Test.Utilities
             process.Start();
             return process;
         }
+        
+        public TestCommand WithEnvironmentVariable(string name, string value)
+        {
+            Environment.Add(name, value);
+            
+            return this;
+        }
     }
 }