Use Stopwatch.StartNew

Use `Stopwatch.StartNew()` instead of creating one then manually starting it.

test/Polly.Specs/Retry/WaitAndRetrySpecs.cs

@@ -1048,8 +1048,7 @@ public void Should_honour_cancellation_immediately_during_wait_phase_of_waitandr
         int attemptsInvoked = 0;
         Action onExecute = () => attemptsInvoked++;
 
-        Stopwatch watch = new Stopwatch();
-        watch.Start();
+        var watch = Stopwatch.StartNew();
 
         PolicyExtensions.ExceptionAndOrCancellationScenario scenario = new PolicyExtensions.ExceptionAndOrCancellationScenario
         {

test/Polly.Specs/Timeout/TimeoutSpecs.cs

@@ -266,14 +266,12 @@ public void Should_not_throw_when_timeout_is_greater_than_execution_duration__pe
     [Fact]
     public void Should_throw_timeout_after_correct_duration__pessimistic()
     {
-        Stopwatch watch = new Stopwatch();
-
         TimeSpan timeout = TimeSpan.FromSeconds(1);
         var policy = Policy.Timeout(timeout, TimeoutStrategy.Pessimistic);
 
         TimeSpan tolerance = TimeSpan.FromSeconds(3); // Consider increasing tolerance, if test fails transiently in different test/build environments.
 
-        watch.Start();
+        var watch = Stopwatch.StartNew();
         Should.Throw<TimeoutRejectedException>(() => policy.Execute(() => SystemClock.Sleep(TimeSpan.FromSeconds(10), CancellationToken)));
         watch.Stop();
 
@@ -410,15 +408,13 @@ public void Should_not_throw_when_timeout_is_greater_than_execution_duration__op
     [Fact]
     public void Should_throw_timeout_after_correct_duration__optimistic()
     {
-        Stopwatch watch = new Stopwatch();
-
         TimeSpan timeout = TimeSpan.FromSeconds(1);
         var policy = Policy.Timeout(timeout);
         var userCancellationToken = CancellationToken;
 
         TimeSpan tolerance = TimeSpan.FromSeconds(3); // Consider increasing tolerance, if test fails transiently in different test/build environments.
 
-        watch.Start();
+        var watch = Stopwatch.StartNew();
 
         // Delegate observes cancellation token, so permitting optimistic cancellation.
         Should.Throw<TimeoutRejectedException>(() => policy.Execute(ct => SystemClock.Sleep(TimeSpan.FromSeconds(10), ct), userCancellationToken));