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

This test has one test case which is marked as an integration test,
and the other is not an integration test, but was using the integration
cli. The integration cli does not work correctly if not run in
integration mode.

This was causing an error locally in conftest.py when trying to
create a tmpdir inside a nonexisting integration cache directory.

This commit also ensures that if we try to use the 'old' commands,
BuildStream will fail and instruct the user to use the new command.

This was just deadcode, it's not really used here.

Adds helper function _buildtree_pull_required() to determine if a
pullqueue should be constructed, for commands outside of bst pull
and build where it is determined that an element's buildtree
artifact is to be required given the respective semantics and
config. Utilised in push() to attempt to mitigate skipping the push
of partial elements without the user having to have preceded it with
an explicit pull.

cli.py: Add new behaviour to push command description

element.py: Move _cached_buildtree() to be non local private method,
use _KeyStrength types to reduce duplication.

tests/integration/pullbuildtrees.py also updated to cover this
use-case.

The addition of cached build trees being included in element
artifacts has led to mostly redundant download overheads when
pulling from a remote artifact server. As such the default behaviour
of pull shouldn't fetch the build tree object if available.

element.py: extend relevant pull logic for specified subdir
consideration and ensure push logic does not lead to partial
artifact pushes. Change assumption that the buildtree can be
extracted if artifact is cached. __cached_buildtree() and
__pull_directories() helpers added.

_artifactcache/: artifactcache.py & cascache.py inclusion of
helper functions for subdir artifact checking & fetching, fetch
logic extended to only pull required artifact directories.
extract & checkout updated to handle full/partial operation.

tests/: addition of integration test pullbuildtrees.py,
buildtrees.py adapted cli options, testutils/artifactshare.py
has_artifact changed to return artifact digest on true condition.