Add Option to Enable Test Targets

[threeal]

This issue suggests adding a MY_MKDIR_ENABLE_TESTS option to optionally enable or disable test targets in the sample project. This option is proposed as a replacement for the BUILD_TESTING variable, which enables or disables test targets globally in all projects. Having a project-specific option allows users of the sample project to determine whether to enable or disable test targets. This option also follows the convention of the MY_MKDIR_ENABLE_INSTALL option, which enables or disables install targets in the sample project.