Include flexparser.testsuite in the list of packages

[avalentino]

• Closes # (insert issue number)
• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• Added an entry to the CHANGES file

When I build the package in debian sid I get the following warning:

/usr/lib/python3/dist-packages/setuptools/command/build_py.py:215: _Warning: Package 'flexparser.testsuite' is absent from the `packages` configuration.
!!

        ********************************************************************************
        ############################
        # Package would be ignored #
        ############################
        Python recognizes 'flexparser.testsuite' as an importable package[^1],
        but it is absent from setuptools' `packages` configuration.

        This leads to an ambiguous overall configuration. If you want to distribute this
        package, please make sure that 'flexparser.testsuite' is explicitly added
        to the `packages` configuration field.

        Alternatively, you can also rely on setuptools' discovery methods
        (for example by using `find_namespace_packages(...)`/`find_namespace:`
        instead of `find_packages(...)`/`find:`).

        You can read more about "package discovery" on setuptools documentation page:

        - https://setuptools.pypa.io/en/latest/userguide/package_discovery.html

        If you don't want 'flexparser.testsuite' to be distributed and are
        already explicitly excluding 'flexparser.testsuite' via
        `find_namespace_packages(...)/find_namespace` or `find_packages(...)/find`,
        you can try to use `exclude_package_data`, or `include-package-data=False` in
        combination with a more fine grained `package-data` configuration.

        You can read more about "package data files" on setuptools documentation page:

        - https://setuptools.pypa.io/en/latest/userguide/datafiles.html


        [^1]: For Python, any directory (with suitable naming) can be imported,
              even if it does not contain any `.py` files.
              On the other hand, currently there is no concept of package data
              directory, all directories are treated like packages.
        ********************************************************************************

!!
  check.warn(importable)

This PR fixes the warning.

[musicinmybrain]

I suspect it would be even nicer not to install the test suite, but I don’t know what fix to suggest given pypa/setuptools#3260.

[avalentino]

IMHO if you do not intend to install the tests then it is better to move them to a separate tests folder outside the main package.
In this way it is easier to manage and also it is easier to run unittests on the installed package.

[hgrecco]

I like when the testsuite is included in the package so you can test easily that everything is fine for your particular installation.