[API.Tests] Fix IDE0200 warnings (#7013)

Author: martincostello

test/OpenTelemetry.Api.Tests/Trace/TracerTests.cs

@@ -608,7 +608,7 @@ public void StartSpan_WhenCurrentActivityStoppedOnAnotherThread_DoesNotLeaveNewS
         var previousActivity = Activity.Current;
         Assert.NotNull(previousActivity);
 
-        var thread = new Thread(() => previousActivity.Stop());
+        var thread = new Thread(previousActivity.Stop);
         thread.Start();
         thread.Join();
 
@@ -645,7 +645,7 @@ public void StartSpan_WhenCurrentActivityStoppedOnAnotherThread_RestoresFirstRun
         Assert.Same(grandparentActivity, parentActivity.Parent);
         Assert.Same(parentActivity, currentActivity.Parent);
 
-        var thread = new Thread(() => currentActivity.Stop());
+        var thread = new Thread(currentActivity.Stop);
         thread.Start();
         thread.Join();
 
@@ -713,7 +713,7 @@ public void StartRootSpan_WhenCurrentActivityStoppedOnAnotherThread_DoesNotLeave
         var previousActivity = Activity.Current;
         Assert.NotNull(previousActivity);
 
-        var thread = new Thread(() => previousActivity.Stop());
+        var thread = new Thread(previousActivity.Stop);
         thread.Start();
         thread.Join();