tests/artifactshare: safer cleanup_on_sigterm use

See merge request BuildStream/buildstream!1673

Use the documented path [1] to `pytest_cov.embed.cleanup_on_sigterm()`,
to avoid crashing on some versions.

It turns out that pytest_cov v2.6.1 on my machine doesn't like the way
that we were accessing cleanup_on_sigterm(). Access it in such a way
that we will either get the function or an ImportError, as per the
documentation.

[1]: https://pytest-cov.readthedocs.io/en/latest/subprocess-support.html

Remove a couple of cases where it's possible to make the main test
process hang, waiting for something to appear on a queue.

Raise a friendlier exception, earlier, if there was a problem starting
the server process.

Reimplement contains_directory() with FetchTree()

See merge request BuildStream/buildstream!1642

This is required for LRU cache expiry.

If the sources of an element are cached, they will stay cached for the
duration of the bst session.