Since ArtifactCache.setup_remotes() can be expensive and should only
happen once, this commit will assert() if it is called a second time
on an artifact cache instance.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

The initialization of remotes is done by ArtifactCache.setup_remotes()
and as such it was wrong for these tests to be calling
CASCache.initialize_remotes() a second time.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

The SandboxRemote used to construct its own CASCache which was
considered dangerous.  This patch replaces that with acquisition of
the cache via the Platform singleton, hopefully eliminating issues
from having more than one artifact cache object in a single process.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

sandbox/_sandboxremote.py: Acquire cache via Platform

See merge request !797