As discussed over the mailing list, reformat code using Black. This is a
one-off change to reformat all our codebase. Moving forward, we
shouldn't expect such blanket reformats. Rather, we expect each change
to already comply with the Black formatting style.

tests/plugins was missing an __init__.py, which meant pylint
was never run there.

- Rename plugintestutils to testing.

- Don't run the tests from bst-plugins-template. This imports
  buildstream.plugintestutils so will have to be disabled to get
  through CI. This can be re nabled once bst-plugins-template has been
  patched.

Replace some popular copypasta.

This important-looking invocation:

    os.path.join(datafiles.dirname, datafiles.basename)

is equivalent to this shorter invocation:

    project = str(datafiles)

It seems like it's very popular copypasta, replace it with the shorter
one thus:

    # Use 'gsed' or 'sed' etc. as appropriate for your system.
    git config --global alias.sub '!f() { git grep --name-only --null "$1" | gxargs --null gsed --in-place --expression "s/$1/$2/g" ; }; f'
    git sub 'os.path.join(datafiles.dirname, datafiles.basename)' 'str(datafiles)'

A plugin's deprecation warning may be silenced by a project by adding
the plugin to the list 'supress-deprecation-warnings' in the project's
project.conf

Now that the remaining test "filter.py" in the plugins directory
tests a specific element, it makes sense to create a place for
testing elements, just like we do for sources.

This tests exactly the same thing that is tested in
tests/internals/pluginfactory.py (the new location
of tests/plugins/basics.py).