module testing files?

Seeing current modules/* 's files.

Some of the modules have testing files in the directory.
Are the testing files still actually used for the modules?
I assume that only modules/foo/foo.yaml file is required. Other files
like "sources" are meaningless, right?
I think "sources" file is wrongly there.

I just checked my local downloaded modules/* directories.

$ ls */Dockerfile
memcached/Dockerfile mongodb/Dockerfile php/Dockerfile ruby/Dockerfile
$ ls */Makefile
flatpak-runtime/Makefile memcached/Makefile mongodb/Makefile
php/Makefile platform/Makefile ruby/Makefile
$ ls -d */tests
memcached/tests/ mongodb/tests/ php/tests/ ruby/tests/
$ ls -d */sources
eog/sources mariadb/sources mongodb/sources
perl/sources platform/sources python2/sources ruby/sources
flatpak-runtime/sources memcached/sources nodejs/sources
php/sources postgresql/sources python3/sources varnish/sources

I like to see the file structure and testing module is documented or
linked in below page
<a href="" title=""></a>