tests/topotests: Set -o cache_dir=/tmp

tests/topotests: Set -o cache_dir=/tmp

We are mounting the topotests from the host into the container
and mark them as read-only to avoid any modifications to the host.

Pytest tries to create a .pytest_chache directory in that location
which fails because of the read-only mount.

Configure a different cache dir which is writable to resolve this.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>