c++ - How to solve the error LNK2019: unresolved external symbol - function?

I get this error, but I don't know how to fix it.

I'm using Visual Studio 2013. I made the solution name MyProjectTest
This is the structure of my test solution:

-function.h

#ifndef MY_FUNCTION_H
#define MY_FUNCTION_H
int multiple(int x, int y);
#endif

-function.cpp

#include "function.h"
int multiple(int x, int y){
    return x*y;
}

-main.cpp

#include 
#include 
#include "function.h"
using namespace std;
int main(){
    int a, b;
    cin >> a >> b;
    cout << multiple(a, b) << endl;
    system("pause");
    return 0;
}

I'm a beginner; this is a simple program and it runs without error.
I read in the internet and became interested in the unit test, so I created a test project:

File > New > Project... > Installed > Templates > Visual C++ > Test > Native Unit Test Project >

Name: UnitTest1
Solution: Add to solution
Then the location auto-switched to the path of the current open solution
This is the folder structure of the solution:

I only edited file unittest1.cpp:

#include "stdafx.h"
#include "CppUnitTest.h"
#include "../MyProjectTest/function.h"
using namespace Microsoft::VisualStudio::CppUnitTestFramework;
namespace UnitTest1
{
    TEST_CLASS(UnitTest1)
    {
    public:
        TEST_METHOD(TestEqual)
        {
            Assert::AreEqual(multiple(2, 3), 6);
            // TODO: Your test code here
        }
    };
}

But I get error LNK2019: unresolved external symbol.
I know that the implementation of function multiple is missing.
I tried to delete the function.cpp file and I replaced the declaration with the definition, and it run. But writing both declaration and definition in the same file is not recommended.
How can I fix this error without doing that? Should I replace with #include "../MyProjectTest/function.cpp" in file unittest.cpp?
(I'm not good at English very much. Thanks)