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

 * 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.

This patch mitigates the granularity of the mtimes used on the default
gitlab runners, Allowing the test suite to pass on these runners.

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

This adds a new test for parsing artifact cache configuration, which
calls the helper function from the 'artifactcache' module directly
rather than trying to assert based on blind push and pull commands
whether or not we got the complex precedence rules exactly right.

This means frontend push/pull tests no longer need to be so thorough
about testing precedence but they are instead expanded to assert that
multiple caches work correctly.

This extends the 'artifacts' configuration block such that a list of
`url` mappings can be given instead of a single entry. For example:

    artifacts:
      - url: http://example.com/artifacts1
      - url: ssh://ostree@example.com/artifacts2

The OSTreeCache class is updated to set up multiple remotes and query
remote refs from all of them.

There are no automated tests for this yet.

Empty URLs ('') now raise an exception. They cause breakages internally
if we allow them through, and they can only occur if the user or our
tests are misconfiguring things somehow.

We report failure to fetch from the cache by printing a message to
stderr for now. This is because BuildStream's actual logging
functionality can't be used during frontend init -- see issue #168.

So we can create artifact shares and use them easily
in test cases for pushing and pulling artifacts from a share.