Use better temp directory cleanup in tests

Closes #6233

src/Compilers/Server/VBCSCompilerTests/EndToEndDeterminismTest.cs

@@ -21,10 +21,9 @@ public class EndToEndDeterminismTest: TestBase
         /// <returns> An array of bytes that were read from the compiled DLL</returns>
         private byte[] CompileAndGetBytes(string source, string additionalFlags, out string finalFlags)
         {
-            var tempRoot = new TempRoot();
 
             // Setup
-            var tempDir = tempRoot.CreateDirectory();
+            var tempDir = Temp.CreateDirectory();
             var srcFile = tempDir.CreateFile("test.cs").WriteAllText(source).Path;
             var outFile = srcFile.Replace("test.cs", "test.dll");