Add C++ test coverage badge in the README.

README.md

-![FAµST Logo](./gen_doc/images/logo.png)  
 [![pipeline status](https://gitlab.inria.fr/faustgrp/faust/badges/hakim_branch/pipeline.svg?ignore_skipped=true)](https://gitlab.inria.fr/faustgrp/faust/commits/hakim_branch)
 ![pyfaust test coverage](https://gitlab.inria.fr/faustgrp/faust/badges/hakim_branch/coverage.svg?job=pyfaust_test_code_coverage&key_text=pyfaustcov)
 ![matfaust test coverage](https://gitlab.inria.fr/faustgrp/faust/badges/hakim_branch/coverage.svg?job=matfaust_test_code_coverage&key_text=matfaustcov)
+![C++ test coverage](https://gitlab.inria.fr/faustgrp/faust/badges/hakim_branch/coverage.svg?job=ctest&key_text=cppcov)  
 
-# FAuST Toolbox -- Flexible Approximate Multi-Layer Sparse Transform
+# ![FAµST](./gen_doc/images/logo.png) Toolbox -- Flexible Approximate Multi-Layer Sparse Transform
 
 
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