Integration tests dont work by default, and write outside of repo

When running the following command, integration tests dont work:

./setup.py test --addopts '--integration tests/integration/shell.py'

This results in tests trying to write to the tmpfs in /tmp on my host, where ostree fails to set extended attributes.

Which leads to the second part of this, that the integration tests leave behind debris on the host by default; that should not happen.

I later discovered the env var INTEGRATION_CACHE, which fixes things when setting it to the tmp/ subdir of BuildStream repo.

Instead of defaulting to a place which likely wont work and leaving behind debris, it's better to default to somewhere local to the repository. If it's desirable to set INTEGRATION_CACHE to somewhere else, for performance reasons; this should be documented in the HACKING.rst file.