[FEATURE][DEVELOP] Introduce UnitTest

[iwatake2222]

The reason WHY this new feature is needed

• It's important to test each logic to improve quality and prevent degradation

Describe WHAT this new feature is

Add Unit Test

HOW to achieve this new feature

• Use GoogleTest
• Create test directory under desktop directory
• Under the test directory, create the same directory structure as the project. Then add test code for each sub module
• To contain GoogleTest, we use git submodule, rather than FetchContent_Declare function in git
  • It's because, we want to avoid multiple download

Related issues

• Parent issue
  • [FEATURE][DEVELOP] Create a blank project to use OpenGL + GLFW #3
• Related issue
  • None

Additional context

None