For parameters that are required as part of an API, prefix them
by "_" to make it clear they are unused

This allows tests importing them to have access to the commands
directly and removes the need from calling this multiple times

This also fixes multiple bugs where the paths of the tests runners
might not be canonical and tools would not get found correctly

This sits in Context allowing artifact cache to check the cas quota
while not being used for CASServer.

A lot of code that checks cache quota has been touched.

Part of #870

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 unix specification for unix sockets only allocates ~100 characters
for the length of the path. This may be longer than the path for a CI
runner, or the path to a user's buildstream directory.

We use output of `git --version` to determine if we can run some tests
that rely on features from newer git versions. Usually, we expect the
output to be like:

    git version 2.17.2

On some platforms, like MacOS, there could be a suffix after the version
string, so that it looks something like:

    git version 2.17.2 (Apple Git-113)

This causes things to fail like so:

    ValueError: invalid literal for int() with base 10: '2 (Apple Git-113)\n'

Fix logic around `HAVE_OLD_GIT` such that we split the output of
`git --version` without limit on how many times we split. Previously we
used to split only twice so the suffixes like `(Apple Git-113)` are not
part of the parsed version.