[hal] Fix unfinished/incorrect GetCPUTemp functions (#5598)

wpilibj/src/test/java/edu/wpi/first/wpilibj/simulation/RoboRioSimTest.java

@@ -209,6 +209,23 @@ class RoboRioSimTest {
     }
   }
 
+  @Test
+  void testCPUTemp() {
+    RoboRioSim.resetData();
+
+    DoubleCallback callback = new DoubleCallback();
+
+    try (CallbackStore cb = RoboRioSim.registerCPUTempCallback(callback, false)) {
+      final double kCPUTemp = 100.0;
+
+      RoboRioSim.setCPUTemp(kCPUTemp);
+      assertTrue(callback.wasTriggered());
+      assertEquals(kCPUTemp, callback.getSetValue());
+      assertEquals(kCPUTemp, RoboRioSim.getCPUTemp());
+      assertEquals(kCPUTemp, RobotController.getCPUTemp());
+    }
+  }
+
   @Test
   void testSerialNumber() {
     RoboRioSim.resetData();