Fix ServiceController tests (#26208)

* Fix incorrect log name calculation; clear log on start, as before

* Redundant exists check

Author: danmoseley

src/System.ServiceProcess.ServiceController/tests/System.ServiceProcess.ServiceController.TestService/TestService.cs

@@ -33,8 +33,8 @@ public TestService(string serviceName)
 
         public static string GetLogPath(string serviceName)
         {
-            return Assembly.GetEntryAssembly().Location + "." + serviceName + ".log";
-        }        
+            return typeof(TestService).Assembly.Location + "." + serviceName + ".log";
+        }
 
         protected override void OnContinue()
         {
@@ -74,6 +74,8 @@ protected override void OnShutdown()
 
         protected override void OnStart(string[] args)
         {
+            File.Delete(GetLogPath(ServiceName));
+
             WriteLog(nameof(OnStart) + " args=" + string.Join(",", args));
             base.OnStart(args);
         }