TST/API: test column indexing copy/view semantics
This is adding some tests for current behaviour related to the discussions in #33457 and #35417 (and also adds the tests from https://github.com/pandas-dev/pandas/pull/35266 which got never merged). I think it is good to have some more explicit tests of current behaviour, while we are considering changing things.

Note: some semantics I am testing are certainly debatable, but I was experimenting a bit with what is happening on current master.
cc @jbrockmendel @TomAugspurger @jreback  > some semantics I am testing are certainly debatable, but I was experimenting a bit with what is happening on current master.

I'm generally positive on this PR, lean towards only testing explicitly-desired behavior > lean towards only testing explicitly-desired behavior

And do you find the currently tested behaviours desired? This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. @jorisvandenbossche can you merge master and address comments @jorisvandenbossche closing as stale. reopen when ready.