[wpilibc] Rename SpeedControllerGroupTest.cpp (#4258)

Also move motor controller test files into motorcontrol folder to match
Java.

wpilibc/src/test/native/include/motorcontrol/MockMotorController.h

@@ -0,0 +1,25 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#include "frc/motorcontrol/MotorController.h"
+
+namespace frc {
+
+class MockMotorController : public MotorController {
+ public:
+  void Set(double speed) override;
+  double Get() const override;
+  void SetInverted(bool isInverted) override;
+  bool GetInverted() const override;
+  void Disable() override;
+  void StopMotor() override;
+
+ private:
+  double m_speed = 0.0;
+  bool m_isInverted = false;
+};
+
+}  // namespace frc