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

The current directory isn't always in the python module search path,
so we have to ensure it is for the script to work.

Strictly speaking, the user may already have a modified PYTHONPATH
at which point PYTHONPATH=".${PYTHONPATH+:$PYTHONPATH}" is necessary,
but it's probably premature to overcomplicate the documentation like that
before we discover it's a problem.

When cache key tests fail, and when it is intentional that
the keys have changed, then run ./tests/cachekey/update.py
to update the keys.

This is also useful whenever adding new tests.