systests: cp test: lots of cleanup

cp tests flake constantly under VFS (discovered in containers#20161),
and the way these tests were written makes it very, very hard
to understand failures.

This is a (sorry) hard-to-review cleanup:

 - use distinctive container names, not just "cpcontainer"
 - add distinctive test names (e.g. RUNNING vs CREATED)
 - remove unnecessary code
 - remove --pause=false (option is deprecated and, IIUC, a NOP)
 - clean up some confusing slashes in paths
 - "dot notation" tests:
   - add a comment linking to issue, because that's a weird one
     that makes no sense whatsoever
   - fix tests, because they were actually not testing

This cleanup has been tested repeatedly in 20161, I'm just bringing
it into main because 20161's future is uncertain.

Signed-off-by: Ed Santiago <[email protected]>