Reenable test_all, fix _aliases.__all__

[ev-br]

closes gh-283

• reenable test_all
• fix common._aliases.__all__ which was wrong
• make it easier to run self-tests locally, esp if not all optional dependencies are available.

So the __all__ list was wrong for a while, but it was not caught be tests because:

• locally, self-tests required all optional dependencies to be installed, so I never ran the whole $ pytest tests/ locally
• on CI, the problem was not caught either, because the test iterates over sys.modules. So if an implementation module is not imported, it's not checked: this was the case with common._aliases until BUG: torch: fix result_type with python scalars #277 added an unrelated test.

[ev-br]

I think I might want to merge this PR sooner than later, if only to restore the state of the tests and make the CI green by default. Please speak up if you want some more time to review.

[lucascolley]

thanks Evgeni! Just one comment