core: add warnings for Windows

SITL autostart is not working on Windows, so we might as well warn.
Also, it's good practice to use they keyword override.

core/integration_test_helper.h

@@ -9,7 +9,7 @@
 class SitlTest : public testing::Test
 {
 protected:
-    virtual void SetUp()
+    virtual void SetUp() override
     {
 #ifndef WINDOWS
         const int ret = system("./start_px4_sitl.sh");
@@ -18,10 +18,11 @@ class SitlTest : public testing::Test
             abort();
         }
         // We need to wait a bit until it's up and running.
-        std::this_thread::sleep_for(std::chrono::seconds(3));
+#else        std::this_thread::sleep_for(std::chrono::seconds(3));
+        dronecore::LogErr() << "Auto-starting SITL not supported on Windows.";
 #endif
     }
-    virtual void TearDown()
+    virtual void TearDown() override
     {
 #ifndef WINDOWS
         // Don't rush this either.
@@ -31,6 +32,8 @@ class SitlTest : public testing::Test
             dronecore::LogErr() << "./stop_px4_sitl.sh failed, giving up.";
             abort();
         }
+#else
+        dronecore::LogErr() << "Auto-starting SITL not supported on Windows.";
 #endif
     }
 };