Docs: Merge yosys_source into extending_yosys

Move abc_flow content into synthesis/abc document.

docs/source/yosys_internals/extending_yosys/test_suites.rst

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+Testing Yosys
+=============
+
+.. todo:: more about the included test suite
+
+Automatic testing
+-----------------
+
+.. only:: html
+
+   The `Yosys Git repo`_ has automatic testing of builds and running of the
+   included test suite on the following platforms:
+
+   - Ubuntu |test-linux|
+   - macOS |test-macos|
+
+.. _Yosys Git repo: https://github.com/YosysHQ/yosys
+
+.. |test-linux| image:: https://github.com/YosysHQ/yosys/actions/workflows/test-linux.yml/badge.svg?branch=main
+.. |test-macos| image:: https://github.com/YosysHQ/yosys/actions/workflows/test-macos.yml/badge.svg?branch=main
+
+For up to date information, including OS versions, refer to `the git actions
+page`_.
+
+.. _the git actions page: https://github.com/YosysHQ/yosys/actions