tests: copy on write checks for objectstore.Object

Verify the copy on write semantics of `objectstore.Object`, i.e.
content will only be copied at the moment a client wants to write
to `Object`. This also checks that `Object.base` works.

Modify the CI to execute the unit tests in a privileged container
because `Object.read()` works internally by bind mounting a path.
The mount operation needs at least CAP_SYS_ADMIN and overwriting
the file permissions CAP_DAC_OVERRIDE.

.github/workflows/tests.yml

@@ -35,6 +35,7 @@ jobs:
     runs-on: ubuntu-latest
     container:
       image: docker.io/library/python:3.7
+      options: --privileged  # Needed for bind mounts in unit tests
     steps:
       - name: Clone repository
         uses: actions/checkout@v2