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.

- 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.

Pylint can't know that pytest's fixtures are used in a file and
therefore reports false positives. Silencing all those errors

Pylint doesn't play well with pytest fixtures, we therefore need to
silence this error.

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

Hide all of buildstream's internal exceptions from the API surface.

Attempting to compose a list property would result in an unhandled
exception:

    AttributeError: 'CommentedSeq' object has no attribute 'get'

We now at least detect the situation and produce an exception that
the frontend will report neatly:

    Error loading pipeline: element.bst [line 11 column 4]: Only values of type 'dict' can be composed.

I was getting this error from an attempt to conditionally extend the
sources list:

    sources:
      (?):
        arch == "x86_64":
          - url: http://example.com/x86_64

The correct way to do this is to move the conditional into the parent
dict, e.g.:

    (?):
      arch == "x86_64":
        sources:
          - url: https://example.com/x86_64

It would be nice if the error message could hint at how the user can do
what they want, but it doesn't seem possible in this case.