[wpilibc] Rename SpeedControllerGroupTest.cpp (#4258)

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

wpilibc/src/test/native/cpp/motorcontrol/MockMotorController.cpp

@@ -0,0 +1,31 @@
+// 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.
+
+#include "motorcontrol/MockMotorController.h"
+
+using namespace frc;
+
+void MockMotorController::Set(double speed) {
+  m_speed = m_isInverted ? -speed : speed;
+}
+
+double MockMotorController::Get() const {
+  return m_speed;
+}
+
+void MockMotorController::SetInverted(bool isInverted) {
+  m_isInverted = isInverted;
+}
+
+bool MockMotorController::GetInverted() const {
+  return m_isInverted;
+}
+
+void MockMotorController::Disable() {
+  m_speed = 0;
+}
+
+void MockMotorController::StopMotor() {
+  Disable();
+}