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.

Now that both are equivalent, we can skip the sanitization part before
the yaml call.

- 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.

This needs to be exposed as part of the plugin author facing API so
that plugin authors can define custom repo types which will can be
passed to the set of tests which iterate over multiple source types.

Part of the work towards #944

Cyclic imports can be confusing because the order in which we import
dependencies can make the import fail or not. We should not rely on
ordering of imports for our code. This fixes everywhere possible the
imports and silence explicitely some which are not convenient or would
require big refactors

 * create_element_size()

   Now uses a git Repo object instead of a local source, and
   returns the repo.

 * update_element_size()

   Added this function which can now resize the expected output
   of an element generated with create_element_size(), useful
   to allow testing sized elements with the tracking feature.

The autotools example had to be copied over inside of the junction
example, as referring to a path outside of the project directory is
now disallowed.

New file consisting of functions that allow us to generate elements
on the fly
__init__.py now includes create_element_size()