Simplify Test Files Organization

[threeal]

This pull request resolves #77 by introducing the following changes to simplify test files:

• Separate the AssertionTest.cmake file into Assert.cmake, AssertFatalError.cmake, and AssertExecuteProcess.cmake files.
• Separate the InternalTest.cmake file into InternalFormatMessage.cmake and InternalMockMessage.cmake.
• Move tests to be declared in the root CMakeLists.txt.