Fix .NET 3.5 test build.

Author: drieseng

src/Renci.SshNet.Tests/Classes/Common/ExtensionsTest_Concat.cs

@@ -151,7 +151,8 @@ private void Performance(byte[] first, byte[] second, int runs)
 
             Console.WriteLine(stopWatch.ElapsedMilliseconds);
 
-            stopWatch.Restart();
+            stopWatch.Reset();
+            stopWatch.Start();
 
             for (var i = 0; i < runs; i++)
             {

src/Renci.SshNet.Tests/Classes/Common/ExtensionsTest_IsEqualTo_ByteArray.cs

@@ -161,7 +161,8 @@ private void Performance(byte[] left, byte[] right, int runs)
 
             Console.WriteLine(stopWatch.ElapsedMilliseconds);
 
-            stopWatch.Restart();
+            stopWatch.Reset();
+            stopWatch.Start();
 
             for (var i = 0; i < runs; i++)
             {

src/Renci.SshNet.Tests/Classes/Common/ExtensionsTest_Take_Count.cs

@@ -158,7 +158,8 @@ private void Performance(byte[] value, int count, int runs)
 
             Console.WriteLine(stopWatch.ElapsedMilliseconds);
 
-            stopWatch.Restart();
+            stopWatch.Reset();
+            stopWatch.Start();
 
             for (var i = 0; i < runs; i++)
             {