CMake + Qt + GTest文件链接

我有一个简单的CMake + Qt + GTest项目：

.
├── CMakeLists.txt
├── src
│   ├── CMakeLists.txt
│   ├── main.cpp
│   └── utils
│       ├── calculator.cpp
│       └── calculator.h
└── tests
    ├── calculator_test.cpp
    └── CMakeLists.txt

根CMakeFiles.txt

cmake_minimum_required(VERSION 3.5)

project(random_project)

# Place binaries and libraries according to GNU standards.
include(GNUInstallDirs)
set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/${CMAKE_INSTALL_LIBDIR})
set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/${CMAKE_INSTALL_LIBDIR})
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/${CMAKE_INSTALL_BINDIR})

# Add source folder.
add_subdirectory(src)

# Add GTest.
include(cmake/googletest.cmake)
fetch_googletest(
        ${PROJECT_SOURCE_DIR}/cmake
        ${PROJECT_BINARY_DIR}/googletest
)

enable_testing()

# Add tests folder.
add_subdirectory(tests)

我想达到calculator.h在calculator_test.cpp它采用GTEST：

#include <gtest/gtest.h>
#include "utils/calculator.h" // <- Want to reach this in that path format.

TEST(example, add)
{
    EXPECT_EQ(8, Calculator::sum(3, 5));
}

我尝试链接，但是找不到测试，并且路径是相对的：

测试/CMakeFiles.txt

# Find the Qt libraries.
find_package(Qt5Widgets REQUIRED)

# Qt5 libraries.
set(QT5_LIBRARIES
        Qt5::Widgets)

# Source files.
set(SOURCE_FILES
        calculator_test.cpp
        ../src/utils/calculator.cpp
        ../src/utils/calculator.h
        ../src/main.cpp) # If I link like this, test is not found.

# Tell CMake to create the executable.
add_executable(unit_tests ${SOURCE_FILES})

# Use the Widgets module from Qt5.
target_link_libraries(unit_tests ${QT5_LIBRARIES} gtest_main)

# Add tests.
add_test(
        NAME unit_tests
        COMMAND ${CMAKE_BINARY_DIR}/${CMAKE_INSTALL_BINDIR}/unit_tests
)

如何链接calculator.h在calculator_test.cpp测试用的CMakeLists.txt？