_sandboxremote.py: Add sigterm handler that sends CancelOperation

Closes #725

See merge request !900

This doesn't change the test behavior and enable for better
composability of tests

This will allow for further de-duplication of work.

The unix-logs is always empty, there is therefore no reasons of keeping
it there

This makes all tests use the same template, which makes reasoning
about them simpler

This allows us to make sure that tests behave nicely while missing brwap
and ostree.

This removes the need of having it synchronized in multiple places