push.py: Add tests to test remote cache expiry

The tests include a new environment variable: BST_CACHE_QUOTA.
This variable is used in receive_main() in pushreceive.py.
Test names: test_artifact_expires, test_large_artifact_is_not_cached_remotely
test_recently_pulled_artifact_does_not_expire: marked as xfail until
we implement LRU expiry in remote share