Fix imports

Author: ErikSchierboom

exercises/practice/hamming/.meta/Generator.tpl

@@ -1,3 +1,4 @@
+using System;
 using Xunit;
 
 public class HammingTests

exercises/practice/hamming/HammingTests.cs

@@ -1,3 +1,4 @@
+using System;
 using Xunit;
 
 public class HammingTests

exercises/practice/perfect-numbers/.meta/Generator.tpl

@@ -1,3 +1,4 @@
+using System;
 using Xunit;
 
 public class PerfectNumbersTests

exercises/practice/perfect-numbers/PerfectNumbersTests.cs

@@ -1,3 +1,4 @@
+using System;
 using Xunit;
 
 public class PerfectNumbersTests

exercises/practice/wordy/.meta/Generator.tpl

@@ -1,3 +1,4 @@
+using System;
 using Xunit;
 
 public class WordyTests

exercises/practice/wordy/WordyTests.cs

@@ -1,3 +1,4 @@
+using System;
 using Xunit;
 
 public class WordyTests