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

Providing a remote will limit build's pull/push remote actions to
the given remote specifically, ignoring those defined via user or
project configuration.

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.

When there is less than 2GB left, it cleans up have 10GB available.
These values are configurable.

Adds a test to ensure that BuildStream alerts the user of a skipped push
when the remote already has the artifact cached.

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

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.

The CAS server now supports LRU.