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.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

tests/sandboxes 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)'

For parameters that are required as part of an API, prefix them
by "_" to make it clear they are unused

We want external plugins to be able to make use of the core testing utils.
This commit exposes the basic utilities which are currently in use in
bst-external plugins. If necessary, more utilities could be exposed in the
future.

Moves the following files from tests/testutils/ to
buildstream/plugintestingutils/:
o runcli.py
o integration.py

As part of this, this commit makes the following changes to runcli.py
and integration.py:
o runcli.py: Fix linting errors
o runcli.py: Add user facing documentation
o Integration.py: Add user facing documentation

Before running a command in the sandbox, check its existence and fail
early if it does not.

This fixes issue #289.