Add CMake test infrastructure for FBOSS Image Builder

Summary:
**Pre-submission checklist**
- [X] I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running `pip install -r requirements-dev.txt && pre-commit install`
- [X] `pre-commit run`

Integrate distro_cli Python unit tests into the FBOSS CMake build system to enable automated testing in CI/CD pipelines.

- Add FbossImageDistroCliTests.cmake to discover and register Python unit tests
- Configure test discovery for distro_cli modules (builder, cmds, lib, tools)
- Update root CMakeLists.txt to include distro_cli test suite
- Enable distro_cli tests in GitHub Actions workflow
- Update distro_cli README with build and test instructions

The CMake configuration automatically discovers all *_test.py files and registers them as CTest targets, allowing tests to run via 'ctest' or 'make test'.

X-link: facebook/fboss#797

Reviewed By: KevinYakar

Differential Revision: D91709321

Pulled By: kevin645

fbshipit-source-id: eee2c3541b503d88e46b5eba36dc2ef84fd80bfa

Author: raghav-nexthop

build/fbcode_builder/CMake/FBPythonBinary.cmake

@@ -158,6 +158,8 @@ function(add_fb_python_executable TARGET)
       # CMake doesn't really seem to like having a directory specified as an
       # output; specify the __main__.py file as the output instead.
       set(zipapp_output_file "${zipapp_output}/__main__.py")
+      # Update output_file to match zipapp_output_file for dir type
+      set(output_file "${zipapp_output_file}")
       list(APPEND
         extra_cmd_params
         COMMAND "${CMAKE_COMMAND}" -E remove_directory "${zipapp_output}"