e2e: implement nested container/namespace tests

[dtrudg]

Type of issue

technical debt

Description of issue

We have an increasing number of issues reported related to running singularity nested (in itself, or other runtimes), or within nested namespaces (see e.g. #2698)

The e2e framework doesn't perform nested execution tests, or offer a good way to do this. Ideally we will add additional e2e profiles that would execute singularity nested within an outer container, so that we can then verify expected behaviour comprehensively.